We've missed the dependency to rsync, so build fails on
minimal containers.

Fixes: 59b2bd05 ("kbuild: add 'headers' target to build up uapi headers in usr/include")
Signed-off-by: Enrico Weigelt, metux IT consult <info@metux.net>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>