uapi: export tc_skbmod.h

Fixes commit 735cffe5d800 ("net_sched: Introduce skbmod action")
Not used by iproute2 but maybe in future.
Signed-off-by: Stephen Hemminger <sthemmin@microsoft.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/uapi/linux/tc_act/Kbuild

@@ -12,3 +12,4 @@ header-y += tc_bpf.h
 header-y += tc_connmark.h
 header-y += tc_ife.h
 header-y += tc_tunnel_key.h
+header-y += tc_skbmod.h