libcheck: disable subunit support

libcheck: disable subunit support

We don't have a recipe for subunit in core (or any layer in the layer
index), and unfortunately a bug in the CMakeLists means that the native
build can detect subunit on the host (so host contamination) and only
half-enable it, so the build fails.

Take a patch from upstream's github to add an option for subunit, and
explicitly disable it.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>