ANDROID: kbuild: Copy out-of-tree kernel headers to INSTALL_HDR_PATH

The external modules "headers_install" target does not copy the
out-of-tree kernel headers from external modules obj folders to
INSTALL_HDR_PATH.
Add $(hdr-prefix) as the rsync command source folder prefix to copy
out-of-tree kernel headers.

Bug: 173331163
Signed-off-by: Tai Kuo <taikuo@google.com>
Change-Id: Ie54c64d026acda65894e048a785c0b66cc9a7ddf
Signed-off-by: Matthias Maennich <maennich@google.com>