revert previous

Somehow the compat for __dead is not working properly on macos
(cirrus ci) since it complains that parse_debug() does not return
a value in all control paths when it uses usage() (marked as __dead)
to catch a wrong usage.

gg.c

@@ -305,7 +305,7 @@ get(const char *r)
 	}
 }
 
-static void __dead
+static void __attribute__((noreturn))
 usage(void)
 {
 	fprintf(stderr, "version: " GG_STRING "\n");