xdriver_xf86-video-vmware: only avaiblable on x86

... since BR2_PACKAGE_MESA3D_GALLIUM_DRIVER_SVGA is only available on x86.

Fix warnings like:
  warning: (BR2_PACKAGE_XDRIVER_XF86_VIDEO_VMWARE) selects
  BR2_PACKAGE_MESA3D_GALLIUM_DRIVER_SVGA which has unmet direct dependencies
  (BR2_PACKAGE_MESA3D && (BR2_i386 || BR2_x86_64))

Signed-off-by: Jérôme Pouiller <jezz@sysmic.org>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>