openwrt: add patch to expose unset symbols (#2702)

The GitHub actions pipeline is not interrupted for unset symbols due to
OpenWrt not failing in all cases of unset symbols.

Adda patch to consistently fail for unset symbols.

Ref #2701 (build process is waiting for user input)

Signed-off-by: David Bauer <mail@david-bauer.net>

Signed-off-by: David Bauer <mail@david-bauer.net>