kbuild: remove redundant -rR flag of hdr-inst

Passing -rR for "make headers_install" is redundant because
the top Makefile has already set -rR to MAKEFLAGS.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Signed-off-by: Michal Marek <mmarek@suse.cz>