libobs: Fix leaking obs-internal.h

Removes prior attempt to expose libcaption headers which really shouldnt
have public. This instead moves the obs-internal include out of the
public obs-scene.h and into it's implementation.

libobs/obs-scene.c

@@ -20,6 +20,7 @@
 #include "util/util_uint64.h"
 #include "graphics/math-defs.h"
 #include "obs-scene.h"
+#include "obs-internal.h"
 
 const struct obs_source_info group_info;