Dockerfile: Add missing "chmod" of u-boot-gen-combined

I had added this line locally, rebuild the image, but didn't ensure that
I had committed the correct version of the patch as well.

Fixes: 75b031ee4a96 ("Dockerfile: download binaries for Nokia RX-51")
Signed-off-by: Tom Rini <trini@konsulko.com>
1 file changed