Kbuild : remove header-y from uapi kbuild

header-y support is not required after
("uapi: export all headers under uapi directories") so
remove header-y from include/uapi/linux/Kbuild.

Change-Id: I0a3348f40563285b5209ccd5dfbf512d9cf096b9
Signed-off-by: Naitik Bharadiya <bharad@codeaurora.org>