hv: Add hyperv.h to uapi headers

This patch adds the hyperv.h header to the uapi folder, and adds it to the Kbuild file.
Doing this enables compiling userspace Hyper-V tools using the installed headers.

Version 2: Split UAPI parts into new header, instead of duplicating.

Signed-off-by: Bjarke Istrup Pedersen <gurligebis@gentoo.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>