avutil/internal: Move avpriv-file API to a header of its own

It is not used by the large majority of files that include
lavu/internal.h.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>

libavutil/random_seed.c

@@ -35,6 +35,7 @@
 #include <time.h>
 #include <string.h>
 #include "avassert.h"
+#include "file_open.h"
 #include "internal.h"
 #include "intreadwrite.h"
 #include "timer.h"