scripts/Makefile.headersinst: install staging UAPI header files to usr

Currently when UAPI headers for a staging driver are kept under the
drivers/staging directory, `make headers_install' installs them to
<install_prefix>/drivers instead of the usual <install_prefix>/usr. This
is a non-standard and unexpected location. Fix this by doing the
necessary string substitutions so that the UAPI headers for staging
drivers end up in <install_prefix>/usr/include/linux/staging.

Change-Id: Ibf6b813e69eea78fc59a4a505c75c6093eca70dd
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>