redhat/kernel.spec.template: fix rtonly build
rtonly build now no longer disables cross_headers, which fail on headers_install due to missing rsync.
Install rsync if either headers or cross_headers are enabled.
Fixes: 18b77b81 ("redhat/kernel.spec.template: enable cross for base/RT") Signed-off-by: Jan Stancek jstancek@redhat.com