meson.build: link the sources directly into libxkbcommon-x11

meson.build: link the sources directly into libxkbcommon-x11

Similar to 75ce741ab97e3d17a0c9b06dd4bdf57c00d5538e, just for the -x11
sublibrary.

This works around meson bug 3937, 'link_whole' arguments don't get added into
the final static library and we end up with a virtually empty 8-byte
libxkbcommon-x11.a file, see https://github.com/mesonbuild/meson/issues/3937

The internal lib is still built for the one test case that requires it.

Fixes #86

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>