uapi: Add UAPI headers for slatecom_interface driver

Add slatecom_interface header file in gen_headers to make it
accessible from userspace modules who uses Android.bp files
for compilation.

Change-Id: Ie298ec28983c16999d941ee8667e2dd7b5c3db22
Signed-off-by: Kiran Gunda <kgunda@codeaurora.org>