mesa3d-etna_viv: Don't include X11 headers if X11 is not available
Revert "mesa3d-etna_viv: Add dependency towards xlib_libXxf86vm"

The previous commit (447ed5b1b14655ee3a278796848d158bfd5311c2)
makes sure the X11 headers are not included if X11 is available.
As a result, we can now build mesa3d without any of the
X11 libraries.

This reverts commit 8ac7f88c0f3e6ce6289bfecca75c7e05dc8443db.