Only define detect_mtu() in gnutls code path

Signed-off-by: Nikos Mavrogiannopoulos <nmav@redhat.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>

dtls.c

@@ -1067,6 +1067,8 @@ int dtls_mainloop(struct openconnect_info *vpninfo, int *timeout)
 	return work_done;
 }
 
+#if defined(DTLS_GNUTLS)
+
 /* Old glibc doesn't define that */
 #if defined(__linux__) && !defined(IPV6_PATHMTU)
 # define IPV6_PATHMTU 61
@@ -1331,6 +1333,8 @@ static void detect_mtu(struct openconnect_info *vpninfo)
 	free(buf);
 }
 
+#endif
+
 #else /* !HAVE_DTLS */
 #warning Your SSL library does not seem to support Cisco DTLS compatibility
 #endif