which.c: Fix buildability.

Provides a dummy definition of __FBSDID to avoid compile errors,
without deleting the ID string inherited from FreeBSD.

TESTED:
Builds now work on 10.4, including universal builds where applicable.

tiger_only/src/which.c

@@ -26,6 +26,12 @@
 
 #include <sys/cdefs.h>
 
+/* Modification from original source */
+#ifndef __FBSDID
+#define __FBSDID(x)
+#endif
+/* End of addition */
+
 __FBSDID("$FreeBSD: src/usr.bin/which/which.c,v 1.6 2005/02/10 16:04:22 ru Exp $");
 
 #include <sys/stat.h>