techpack: Fix uapi headers_install

Since commit <d5470d14> ("kbuild: re-implement
Makefile.headersinst without recursion"), the headers_install
excludes Kbuild from providing information regarding uapi headers.
In fact, it recursively pulls all the headers from a given path,
in this case techpack/*. This isn't correct as techpack/ may have
non-uapi headers as well, which shouldn't be considered. Hence,
strictly look for include/uapi/$dir directory for the uapi headers.

Change-Id: Ic7b3f927bc962729da1b87ed90e5f50a1e4cd5c5
Signed-off-by: Raghavendra Rao Ananta <rananta@codeaurora.org>