Fix compilation with GCC and -O0.

Andrei reported an issue with building unit when using '-O0' with GCC
producing the following compiler errors

cc -c -pipe -fPIC -fvisibility=hidden -O -W -Wall -Wextra -Wno-unused-parameter -Wwrite-strings -Wmissing-prototypes -Werror -g -O0  -I src -I build   \
                      \
                     \
-o build/src/nxt_unit.o \
-MMD -MF build/src/nxt_unit.dep -MT build/src/nxt_unit.o \
src/nxt_unit.c
src/nxt_unit.c: In function ‘nxt_unit_log’:
src/nxt_unit.c:6601:9: error: ‘msg’ may be used uninitialized [-Werror=maybe-uninitialized]
 6601 |     p = nxt_unit_snprint_prefix(p, end, pid, level);
      |         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
src/nxt_unit.c:6682:1: note: by argument 2 of type ‘const char *’ to ‘nxt_unit_snprint_prefix’ declared here
 6682 | nxt_unit_snprint_prefix(char *p, const char *end, pid_t pid, int level)
      | ^~~~~~~~~~~~~~~~~~~~~~~
src/nxt_unit.c:6582:22: note: ‘msg’ declared here
 6582 |     char             msg[NXT_MAX_ERROR_STR], *p, *end;
      |                      ^~~
src/nxt_unit.c: In function ‘nxt_unit_req_log’:
src/nxt_unit.c:6645:9: error: ‘msg’ may be used uninitialized [-Werror=maybe-uninitialized]
 6645 |     p = nxt_unit_snprint_prefix(p, end, pid, level);
      |         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
src/nxt_unit.c:6682:1: note: by argument 2 of type ‘const char *’ to ‘nxt_unit_snprint_prefix’ declared here
 6682 | nxt_unit_snprint_prefix(char *p, const char *end, pid_t pid, int level)
      | ^~~~~~~~~~~~~~~~~~~~~~~
src/nxt_unit.c:6625:35: note: ‘msg’ declared here
 6625 |     char                          msg[NXT_MAX_ERROR_STR], *p, *end;
      |                                   ^~~
cc1: all warnings being treated as errors

The above was reproduced with

  $ ./configure --cc-opt=-O0 && ./configure python && make -j4

This warning doesn't happen on clang (15.0.4) or GCC (8.3) and seems to
have been introduced in GCC 11.  The above is from GCC (12.2.1, Fedora
37).

The trigger of this GCC issue is actually part of a commit I introduced
a few months back to constify some function parameters and it seems the
consensus for how to resolve this problem is to simply remove the const
qualifier from the *end parameter to nxt_unit_snprint_prefix().

Reported-by: Andrei Zeliankou <zelenkov@nginx.com>
Link: <https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100417>
Link: <samtools/htslib#1285>
Link: <https://gcc.gnu.org/gcc-11/changes.html>
Fixes: 4418f99 ("Constified numerous function parameters.")
Signed-off-by: Andrew Clayton <a.clayton@nginx.com>

src/nxt_unit.c

@@ -196,7 +196,7 @@ static int nxt_unit_request_hash_add(nxt_unit_ctx_t *ctx,
 static nxt_unit_request_info_t *nxt_unit_request_hash_find(
     nxt_unit_ctx_t *ctx, uint32_t stream, int remove);
 
-static char * nxt_unit_snprint_prefix(char *p, const char *end, pid_t pid,
+static char * nxt_unit_snprint_prefix(char *p, char *end, pid_t pid,
     int level);
 static void *nxt_unit_lvlhsh_alloc(void *data, size_t size);
 static void nxt_unit_lvlhsh_free(void *data, void *p);
@@ -6679,7 +6679,7 @@ static const char * nxt_unit_log_levels[] = {
 
 
 static char *
-nxt_unit_snprint_prefix(char *p, const char *end, pid_t pid, int level)
+nxt_unit_snprint_prefix(char *p, char *end, pid_t pid, int level)
 {
     struct tm        tm;
     struct timespec  ts;