ffmpeg: make q_pressed volatile, it can be changed from signal handlers.

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

ffmpeg.c

@@ -146,7 +146,7 @@ static int audio_volume = 256;
 static int exit_on_error = 0;
 static int using_stdin = 0;
 static int run_as_daemon  = 0;
-static int q_pressed = 0;
+static volatile int q_pressed = 0;
 static int64_t video_size = 0;
 static int64_t audio_size = 0;
 static int64_t extra_size = 0;