xserver_xorg-server: Add optional package xlib_libXcomposite as dependency

Make sure that xlib_libXcomposite is build before xserver_xorg-server.

With this defconfig

BR2_TOOLCHAIN_EXTERNAL=y
BR2_PACKAGE_XORG7=y
BR2_PACKAGE_XSERVER_XORG_SERVER=y
BR2_PACKAGE_XSERVER_XORG_SERVER_MODULAR=y
BR2_PACKAGE_XLIB_LIBXCOMPOSITE=y

and "make xserver_xorg-server" the package xlib_libXcomposite was ignored.

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>