meson: only run vulkan's meson.build when building vulkan

Fixes: d1992255bb "meson: Add build Intel "anv" vulkan driver"
Signed-off-by: Eric Engestrom <eric.engestrom@intel.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Dylan Baker <dylan@pnwbakers.com>

src/meson.build

@@ -52,7 +52,9 @@ subdir('mapi')
 # TODO: opengl
 subdir('compiler')
 subdir('egl/wayland/wayland-drm')
-subdir('vulkan')
+if with_any_vk
+  subdir('vulkan')
+endif
 if with_gallium_radeonsi or with_amd_vk
   subdir('amd')
 endif