openh264: Directly include the deprecation guards header

Make easier to avoid compile failure when reworking the internal
headers.

libavcodec/libopenh264enc.c

@@ -25,6 +25,7 @@
 #include "libavutil/attributes.h"
 #include "libavutil/common.h"
 #include "libavutil/opt.h"
+#include "libavutil/internal.h"
 #include "libavutil/intreadwrite.h"
 #include "libavutil/mathematics.h"