src: Fix a build issue on header inclusion in internal.h

Fixes:
In file included from utils.c:11:0:
./internal.h:93:17: error: unknown type name 'FILE'
 int nft_fprintf(FILE *fp, void *obj, uint32_t type, uint32_t flags, int
 (*snprintf_cb)(char *buf, size_t bufsiz, void *obj, uint32_t type,
 uint32_t flags));

Signed-off-by: Tomasz Bursztyka <tomasz.bursztyka@linux.intel.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>