tweaks: avoid an unused-variable warning when using --disable-comment

src/text.c

@@ -1140,7 +1140,9 @@ bool execute_command(const char *command)
 void discard_until(const undo *thisitem, openfilestruct *thefile)
 {
     undo *dropit = thefile->undotop;
+#ifdef ENABLE_COMMENT
     undo_group *group;
+#endif
 
     while (dropit != NULL && dropit != thisitem) {
 	thefile->undotop = dropit->next;