uapi: fix linux/llc.h userspace compilation error

Include <linux/if.h> to fix the following linux/llc.h userspace
compilation error:

/usr/include/linux/llc.h:26:27: error: 'IFHWADDRLEN' undeclared here (not in a function)
  unsigned char   sllc_mac[IFHWADDRLEN];

Signed-off-by: Dmitry V. Levin <ldv@altlinux.org>
Signed-off-by: David S. Miller <davem@davemloft.net>