Issue title: netmap module on FreeBSD: opt_inet.h file not found
 Issue body: Hi,

I installed 11.2 FreeBSD-RELEASE with source tree and tried to buid netmap module, but it fails with following error:
```
/usr/src/sys/dev/netmap/netmap_freebsd.c:27:10: fatal error: 'opt_inet.h' file not found
#include "opt_inet.h"
```
I must be missing something very obvious since this is the first time I try FreeBSD. 
Could you please help me to find the cause of this error?

Thanks in advance.
 Comments: 
 Comment 0: Closing the issue, since I realized netmap is already installed and loaded on FreeBSD 11-2.
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