libobs: Add VIDEO_CS_SRGB enum value

Can't implement all at once because of AMF submodule.

libobs/media-io/video-io.h

@@ -74,6 +74,7 @@ enum video_colorspace {
 	VIDEO_CS_DEFAULT,
 	VIDEO_CS_601,
 	VIDEO_CS_709,
+	VIDEO_CS_SRGB,
 };
 
 enum video_range_type {