libavcodec/decode: Mark decode_simple_internal() as inline

This was suggested in
https://github.com/google/oss-fuzz/issues/3787
to reduce the grouping errors by oss-fuzz
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/decode.c

@@ -308,7 +308,7 @@ static int64_t guess_correct_pts(AVCodecContext *ctx,
  * returning any output, so this function needs to be called in a loop until it
  * returns EAGAIN.
  **/
-static int decode_simple_internal(AVCodecContext *avctx, AVFrame *frame)
+static inline int decode_simple_internal(AVCodecContext *avctx, AVFrame *frame)
 {
     AVCodecInternal   *avci = avctx->internal;
     DecodeSimpleContext *ds = &avci->ds;