mesa: fix pipe_loader_sw build without libx11

mesa: fix pipe_loader_sw build without libx11

* this is needed to fix pipe_loader_sw build without libx11,
  but building with HAVE_PIPE_LOADER_XLIB enabled still doesn't
  work (nobody is using this and it was removed in newer SRCREV).
  See https://bugs.freedesktop.org/show_bug.cgi?id=66357 for details.

(From OE-Core rev: 0cc7f4ebad7f4390ac340854b57111d5084bd956)

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>
Signed-off-by: Saul Wold <sgw@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>