Fix build with threading disabled

The avcodec_thread_free() compatibility wrapper calls ff_thread_free(),
which is not defined when threading is disabled.  Make this call
conditional.
Signed-off-by: Mans Rullgard <mans@mansr.com>

libavcodec/utils.c

@@ -1297,7 +1297,9 @@ int avcodec_thread_init(AVCodecContext *s, int thread_count)
 
 void avcodec_thread_free(AVCodecContext *s)
 {
+#if HAVE_THREADS
     return ff_thread_free(s);
+#endif
 }
 
 #endif