meson: Only include virtio when DRM available

The unconditional inclusion of the `virtio` subdirectory introduced by !24733 causes a full dependency on DRM, it breaks any systems that have `system_has_kms_drm = false`, including Turnip with just the KGSL KMD. A temporary solution to this is gating the inclusion when `system_has_kms_drm` isn't set but this should be replaced with more specific gating in the future.

Signed-off-by: Mark Collins <mark@igalia.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/26100>

src/meson.build

@@ -91,7 +91,9 @@ endif
 if with_gallium_etnaviv
   subdir('etnaviv')
 endif
-subdir('virtio')
+if system_has_kms_drm
+  subdir('virtio')
+endif
 if with_gallium_freedreno or with_freedreno_vk or with_tools.contains('freedreno')
   subdir('freedreno')
 endif