makefs: add missing break for 'g' case in tarfs test

Reported by:	Coverity Scan
CID:		1506954
Fixes: 6cb78fa479 ("tarfs: Repeat tests using GNU tar if available.")
Sponsored by:	The FreeBSD Foundation

tests/sys/fs/tarfs/mktar.c

@@ -174,6 +174,7 @@ main(int argc, char *argv[])
 		switch (opt) {
 		case 'g':
 			opt_g = true;
+			break;
 		case 'v':
 			opt_v = true;
 			break;