libgtop/metacity: add x11 to required DISTRO_FEATURES

The recipes depend on virtual/libx11 indirectly, they only can be
built with x11 in DISTRO_FEATURES.

Signed-off-by: Wenzong Fan <wenzong.fan@windriver.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>