Fix FTBS of sdk due to missing umock-c package when installing umock-c-dev

Fix FTBS of sdk due to missing umock-c package when installing umock-c-dev

umock-c-dev has a built-in dependency to umock-c by default.

Previously there was no 'umock-c' package as all was shipped as
headers/cmake in -dev and a -staticdev package only.

ALLOW_EMPTY_${PN} = "1" will enforce the package generation.

This will be discussed with upstream on how to fix the package.

Bug-AGL: SPEC-3391

Signed-off-by: Jan-Simon Möller <jsmoeller@linuxfoundation.org>
Change-Id: I2429ae1fe77e6dcfb630a78ffc420046593b952f
Reviewed-on: https://gerrit.automotivelinux.org/gerrit/c/AGL/meta-agl/+/24565
Reviewed-by: Scott Murray <scott.murray@konsulko.com>
Tested-by: Jenkins Job builder account <agl-jobbuilder@automotivelinux.org>
ci-image-build: Jenkins Job builder account <agl-jobbuilder@automotivelinux.org>
ci-image-boot-test: Jenkins Job builder account <agl-jobbuilder@automotivelinux.org>