meson: only build imgui when needed

Only required for Intel tools or the Vulkan overlay layer.

Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Eric Engestrom <eric.engestrom@intel.com>

src/vulkan/meson.build

@@ -59,6 +59,6 @@ endif
 
 subdir('util')
 subdir('wsi')
-if get_option('vulkan-overlay-layer')
+if with_vulkan_overlay_layer
   subdir('overlay-layer')
 endif