meson: only build vl_winsys_dri.c when x11 platform is used

This seems to have been missed in the move from autotools

This fixes the following build issue:

../src/gallium/auxiliary/vl/vl_winsys_dri.c:34:10: fatal error: X11/Xlib-xcb.h: No such file or directory
 #include <X11/Xlib-xcb.h>
          ^~~~~~~~~~~~~~~~

Fixes: b1b65397
       ("meson: Build gallium auxiliary")
Reviewed-by: Dylan Baker <dylan@pnwbakers.com>
(cherry picked from commit 1d92d648)