BUILD: ssl: strchr definition changed in C23

BUILD: ssl: strchr definition changed in C23

New gcc and clang versions from fedora rawhide seems to use the C23
standard by default. This version changes the definition of some
string.h functions, which now return a const char * instead of a char *.

src/ssl_sock.c: In function ‘SSL_CTX_keylog’:
src/ssl_sock.c:4475:17: error: assignment discards ‘const’ qualifier from pointer target type [-Werror=discarded-qualifiers]
 4475 |         lastarg = strrchr(line, ' ');

Strangely, -Wdiscarded-qualifiers does not seem to catch all the
strrchr.

Should fix issue #3228.

This could be backported in previous versions.