Make it simpler to build netgraph modules outside of the kernel source

tree.  This change follows similar ones in the device tree.

MFC after:	2 weeks

sys/netgraph/netgraph.h

@@ -54,7 +54,9 @@
 #include <sys/module.h>
 #include <sys/mutex.h>
 
+#ifdef HAVE_KERNEL_OPTION_HEADERS
 #include "opt_netgraph.h"
+#endif
 
 /* debugging options */
 #define NG_SEPARATE_MALLOC	/* make modules use their own malloc types */