rtp: fix a warning

In c23, strchr is a type-generic function so it returns a pointer to
const char if the 1st argument is such and not to unqualified char as in
earlier standards.

Author: MartinPulec

src/rtp/rtp.c

@@ -3761,7 +3761,7 @@ bool rtp_set_encryption_key(struct rtp *session, const char *passphrase)
         char *canonical_passphrase;
         u_char hash[16];
         MD5CTX context;
-        char *slash;
+        const char *slash;
 
         check_database(session);
         if (session->encryption_algorithm != NULL) {