meson: allow building dri driver without window system if osmesa is classic

This was already enabled for gallium based osmesa with gallium drivers
in 9d10581897ef7cfa0f6c392e2048cc04357281b9, so do the same for classic
driver with classic osmesa.

Fixes: cbbd5bb889
       ("meson: build classic osmesa")
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>

meson.build

@@ -382,8 +382,8 @@ if with_any_vk and (with_platform_x11 and not with_dri3)
   error('Vulkan drivers require dri3 for X11 support')
 endif
 if with_dri
-  if with_glx == 'disabled' and not with_egl and not with_gbm
-    error('building dri drivers require at least one windowing system')
+  if with_glx == 'disabled' and not with_egl and not with_gbm and with_osmesa != 'classic'
+    error('building dri drivers require at least one windowing system or classic osmesa')
   endif
 endif