gallium: Do not include offscreen winsys if glx is disabled

Offscreen winsys introduced in Mesa 22.2 depends on glx(dri) and libswkmsdri.

The error message is: /usr/bin/ld: src/gallium/auxiliary/libgalliumvlwinsys.a.p/vl_vl_winsys_dri_vgem.c.o: in function vl_vgem_drm_screen_create': vl_winsys_dri_vgem.c:(.text.vl_vgem_drm_screen_create+0x28): undefined reference to kms_dri_create_winsys'

Fixes: 31dcb396 (gallium/vl: Add software winsys and offscreen winsys)

Cc: mesa-stable

Signed-off-by: nyanmisaka nst799610810@gmail.com