All consumers of in_cksum.h have been properly #ifdefed already,

so do not include the file either as it would give as an extra
dependency on INET.

sys/contrib/pf/net/if_pflog.c

@@ -99,9 +99,11 @@ __FBSDID("$FreeBSD$");
 #include <net/pfvar.h>
 #include <net/if_pflog.h>
 
+#ifdef INET
 #ifdef __FreeBSD__
 #include <machine/in_cksum.h>
 #endif
+#endif
 
 #define PFLOGMTU	(32768 + MHLEN + MLEN)