freebsd_network: Avoid clashes between local and system ffs() definition.

src/libs/compat/freebsd_network/compat/sys/libkern.h

@@ -6,9 +6,11 @@
 #define _FBSD_COMPAT_SYS_LIBKERN_H_
 
 
+#include <strings.h>
 #include <sys/cdefs.h>
 #include <sys/types.h>
 
+
 __BEGIN_DECLS
 
 
@@ -26,8 +28,6 @@ static __inline int imin(int a, int b) { return (a < b ? a : b); }
 extern int abs(int a);
 
 
-#define        ffs(x)  __builtin_ffs(x)
-
 static __inline __pure2 int
 ffsl(long mask)
 {