Give a useful error message if someone tries to compile IPFIREWALL into the

kernel without specifying PFIL_HOOKS as well.

sys/netinet/ip_fw_pfil.c

@@ -29,10 +29,14 @@
 #include "opt_ipfw.h"
 #include "opt_ipdn.h"
 #include "opt_ipdivert.h"
+#include "opt_pfil_hooks.h"
 #include "opt_inet.h"
 #ifndef INET
 #error IPFIREWALL requires INET.
 #endif /* INET */
+#if !defined(KLD_MODULE) && !defined(PFIL_HOOKS)
+#error IPFIREWALL requires PFIL_HOOKS.
+#endif
 
 #include <sys/param.h>
 #include <sys/systm.h>