meson_options: Change default target to generic

This a workaround for a meson bug that doesn't let one set options of subprojects if adding them after the initial configure. So our default options need to pass.

You can reproduce the bug with:

git clone gst-build
meson build
ninja -C build

Then either of the following commands fail:

meson configure build -Domx=enabled
meson configure build -Domx=enabled -Dgst-omx:target=generic