lavu/slicethread: return ENOSYS rather than EINVAL in the dummy func

EINVAL is the wrong error code here, since the arguments passed to the
function are valid. The error is that the function is not implemented in
the build, which corresponds to ENOSYS.

libavutil/slicethread.c

@@ -240,7 +240,7 @@ int avpriv_slicethread_create(AVSliceThread **pctx, void *priv,
                               int nb_threads)
 {
     *pctx = NULL;
-    return AVERROR(EINVAL);
+    return AVERROR(ENOSYS);
 }
 
 void avpriv_slicethread_execute(AVSliceThread *ctx, int nb_jobs, int execute_main)