uapi: export nf_log.h

File is in uapi directory but not being copied on
 make install_headers

Fixes commit 4ec9c8fbbc22 ("netfilter: nft_log: complete
NFTA_LOG_FLAGS attr support").

Signed-off-by: Stephen Hemminger <sthemmin@microsoft.com>
Signed-off-by: David S. Miller <davem@davemloft.net>