testing: add missing using statement (envoyproxy#5085)

Without this change, the code is relying on ADL to correctly resolve
DoAll as ::testing::DoAll. Adding the extra using declaration
prevents problems if the Googletest library changes its implementation.

Risk Level: low
Testing: ran affected tests

Signed-off-by: Alex Konradi <akonradi@google.com>

test/common/ssl/ssl_socket_test.cc

@@ -33,6 +33,7 @@
 #include "openssl/ssl.h"
 
 using testing::_;
+using testing::DoAll;
 using testing::Invoke;
 using testing::NiceMock;
 using testing::Return;