tools: ynl: work around stale system headers

The inability to include the uAPI headers directly in tools/
is one of the bigger annoyances of compiling user space code.
Most projects trade the pain for smaller inconvenience of having
to copy the headers under tools/include.

In case of netlink headers I think that we can avoid both.
Netlink family headers are simple and should be self-contained.
We can try to twiddle the Makefile a little to force-include
just the family header, and use system headers for the rest.

This works fairly well. There are two warts - for some reason
if we specify -include $path/family.h as a compilation flag,
the #ifdef header guard does not seem to work. So we need
to throw the guard in on the command line as well. Seems like
GCC detects that the header is different and tries to include
both. Second problem is that make wants hash sign to be escaped
or not depending on the version. Sigh.

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>