ffplay: signal seek event to read thread

Improves seek delay.
Signed-off-by: Marton Balint <cus@passwd.hu>

ffplay.c

@@ -1184,6 +1184,7 @@ static void stream_seek(VideoState *is, int64_t pos, int64_t rel, int seek_by_by
         if (seek_by_bytes)
             is->seek_flags |= AVSEEK_FLAG_BYTE;
         is->seek_req = 1;
+        SDL_CondSignal(is->continue_read_thread);
     }
 }