tools/net: Use include/uapi with __EXPORTED_HEADERS__

Use the local uapi headers to keep in sync with "recently" added #define's
(e.g. SKF_AD_VLAN_TPID).  Refactored CFLAGS, and bpf_asm doesn't need -I.

Fixes: 3f356385 ("filter: bpf_asm: add minimal bpf asm tool")
Signed-off-by: Kamal Mostafa <kamal@canonical.com>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: David S. Miller <davem@davemloft.net>