avcodec/frame_thread_encoder: Use more natural types

Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>

libavcodec/frame_thread_encoder.c

@@ -40,7 +40,7 @@
 typedef struct{
     AVFrame  *indata;
     AVPacket *outdata;
-    int64_t return_code;
+    int       return_code;
     int       finished;
 } Task;