build/meson: fix x11 test/demo compilation with non-standard xcb

build/meson: fix x11 test/demo compilation with non-standard xcb

The x11 tests/demos did not depend on xcb and xcb-xkb directly, only
indirectly through link_with: libxkbcommon_x11_internal. So linking
worked, but the xcb and xcb-xkb cflags were *not* included when
compiling them. So when using xcb installed in a non-standard location,
what would happen is:

- Library will link with custom xcb and compile with custom xcb headers.
- Test will link with custom xcb and compile with system xcb headers (if
  exist, otherwise fail).

Fixes: https://github.com/xkbcommon/libxkbcommon/issues/57
Reported-by: @remexre
Signed-off-by: Ran Benita <ran234@gmail.com>