ffmpeg: free_input_threads requires HAVE_PTHREADS

Hide behind that macro to avoid warning on systems with
!HAVE_PTHREADS.

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

ffmpeg.c

@@ -155,8 +155,9 @@ static struct termios oldtty;
 static int restore_tty;
 #endif
 
+#if HAVE_PTHREADS
 static void free_input_threads(void);
-
+#endif
 
 /* sub2video hack:
    Convert subtitles to video with alpha to insert them in filter graphs.