bhyve: net_backends.c: add missing __FBSDID

Reviewed by:	jhb
MFC after:	2 weeks
Differential Revision:	https://reviews.freebsd.org/D20883

usr.sbin/bhyve/net_backends.c

@@ -34,11 +34,13 @@
  * features) is exported by net_backends.h.
  */
 
+#include <sys/cdefs.h>
+__FBSDID("$FreeBSD$");
+
 #include <sys/types.h>		/* u_short etc */
 #ifndef WITHOUT_CAPSICUM
 #include <sys/capsicum.h>
 #endif
-#include <sys/cdefs.h>
 #include <sys/ioctl.h>
 #include <sys/mman.h>
 #include <sys/uio.h>