libgl-gdi: Fix unused-variable warnings.

libgl-gdi: Fix unused-variable warnings.

src/gallium/targets/libgl-gdi/libgl_gdi.c:59:16: warning: ‘use_swr’ defined but not used [-Wunused-variable]
   59 | static boolean use_swr = FALSE;
      |                ^~~~~~~
src/gallium/targets/libgl-gdi/libgl_gdi.c:58:16: warning: ‘use_llvmpipe’ defined but not used [-Wunused-variable]
   58 | static boolean use_llvmpipe = FALSE;
      |                ^~~~~~~~~~~~

Signed-off-by: Vinson Lee <vlee@freedesktop.org>
Reviewed-by: Eric Anholt <eric@anholt.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6508>