fftools/ffmpeg: remove unncessary casts for *_thread() return values

These functions used to be passed directly to pthread_create(), which
required them to return void*. This is no longer the case, so they can
return a plain int.

fftools/ffmpeg_sched.h

@@ -102,7 +102,7 @@ typedef struct SchedulerNode {
     unsigned                idx_stream;
 } SchedulerNode;
 
-typedef void* (*SchThreadFunc)(void *arg);
+typedef int (*SchThreadFunc)(void *arg);
 
 #define SCH_DSTREAM(file, stream)                           \
     (SchedulerNode){ .type = SCH_NODE_TYPE_DEMUX,           \