lavf: Fix leftovers from the ff_get_buffer patch

The automated script did not perfectly replace all the instances nor
added internal.h in all the files requiring it.

libavcodec/mxpegdec.c

@@ -27,6 +27,7 @@
 
 #include "mjpeg.h"
 #include "mjpegdec.h"
+#include "internal.h"
 
 typedef struct MXpegDecodeContext {
     MJpegDecodeContext jpg;

libavcodec/pthread.c

@@ -573,7 +573,7 @@ static int submit_packet(PerThreadContext *p, AVPacket *avpkt)
                 pthread_cond_wait(&p->progress_cond, &p->progress_mutex);
 
             if (p->state == STATE_GET_BUFFER) {
-                p->result = p-ff_get_buffer(p->avctx, p->requested_frame);
+                p->result = ff_get_buffer(p->avctx, p->requested_frame);
                 p->state  = STATE_SETTING_UP;
                 pthread_cond_signal(&p->progress_cond);
             }

libavcodec/roqvideoenc.c

@@ -60,6 +60,7 @@
 #include "bytestream.h"
 #include "elbg.h"
 #include "mathops.h"
+#include "internal.h"
 
 #define CHROMA_BIAS 1