UAPI: Fix compilation of the wanxl firmware blob.

The wanxl firmware needs access to some bits of UAPI stuff, so the -I flag in
the Makefile needs adjusting to point at the UAPI headers.

Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>