Fix discard const qualifier

Fix discard const qualifier

Since glibc-2.43 and ISO C23, the functions bsearch, memchr, strchr,
strpbrk, strrchr, strstr, wcschr, wcspbrk, wcsrchr, wcsstr and wmemchr
that return pointers into their input arrays now have definitions as
macros that return a pointer to a const-qualified type when the input
argument is a pointer to a const-qualified type.

additional and p pointer returns are only being used for comparisons so declare
them as const, which matches the input variable.

Fixes:
    ../../../lib/x509/hostname-verify.c: In function 'gnutls_x509_crt_check_hostname2':
    ../../../lib/x509/hostname-verify.c:165:17: warning: assignment discards 'const' qualifier from pointer target type [-Wdiscarded-qualifiers]
      165 |             ((p = strchr(hostname, ':')) != NULL ||
          |                 ^
    ../../../lib/x509/ip.c: In function 'gnutls_x509_cidr_to_rfc5280':
    ../../../lib/x509/ip.c:233:11: warning: assignment discards 'const' qualifier from pointer target type [-Wdiscarded-qualifiers]
      233 |         p = strchr(cidr, '/');
          |           ^
    ../../lib/priority.c: In function '_gnutls_resolve_priorities':
    ../../lib/priority.c:2534:20: warning: assignment discards 'const' qualifier from pointer target type [-Wdiscarded-qualifiers]
     2534 |         additional = strchr(ss, ':');
          |                    ^
    ../../lib/str.c: In function '_gnutls_hostname_compare':
    ../../lib/str.c:722:19: warning: assignment discards 'const' qualifier from pointer target type [-Wdiscarded-qualifiers]
      722 |                 p = strrchr(certname, '.');
          |                   ^

Signed-off-by: Rudi Heitbaum <rudi@heitbaum.com>