anv: Link to libX11-xcb only when unneeded
anv: Link to libX11-xcb only when unneeded

The Makefile unconditionally linked libX11-xcb into libvulkan_intel.so.
But it's needed only if HAVE_PLATFORM_X11.

Fixes build of libvulkan_intel.so on Chromium OS, which has no X11
libraries.

Fixes: 71258e9462c ("anv/x11: Add support for Xlib platform")
Cc: Kevin Strasser <6de8fd4fdea896c0ed51709b02692888255d2cbc@intel.com>
Reviewed-by: Jason Ekstrand <68c46a606457643eab92053c1c05574abb26f861@jlekstrand.net>
Reviewed-by: Emil Velikov <f20878473f414da183da19ee25c8722cc1d711e4@collabora.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler