libobs: Make video frame header public

This fixes compilation of third party plugins that need this
header.

libobs/CMakeLists.txt

@@ -273,6 +273,7 @@ set(public_headers
     media-io/audio-io.h
     media-io/frame-rate.h
     media-io/media-io-defs.h
+    media-io/video-frame.h
     media-io/video-io.h
     obs-audio-controls.h
     obs-config.h