[SMS] avoid mktime on NULL

gmtime(NULL) returns NULL at least in glibc and *can not* be used as
time(NULL). Since we compare two time_t values when checking the validity
period this can be replaced by time(NULL)

openbsc/src/gsm_04_11.c

@@ -305,7 +305,7 @@ static unsigned long gsm340_vp_absolute(u_int8_t *sms_vp)
 	unsigned long minutes;
 
 	expires = gsm340_scts(sms_vp);
-	now = mktime(gmtime(NULL));
+	now = time(NULL);
 	if (expires <= now)
 		minutes = 0;
 	else