User applications frequently hit problems when they try to use
the kernel headers directly, rather than the exported headers.

This adds an explicit warning for this case, and points to
a URL holding an explanation of why this is wrong and what
to do about it.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>