ffmpeg: include conditionally the sws_flags variable

The variable is not used when libavfilter is enabled.
Fix the warning:
ffmpeg.c: At top level:
ffmpeg.c:242: warning: ‘sws_flags’ defined but not used

ffmpeg.c

@@ -167,6 +167,8 @@ static int loop_output = AVFMT_NOOUTPUTLOOP;
 static int qp_hist = 0;
 #if CONFIG_AVFILTER
 static char *vfilters = NULL;
+#else
+static unsigned int sws_flags = SWS_BICUBIC;
 #endif
 
 static int intra_only = 0;
@@ -239,8 +241,6 @@ static char *forced_key_frames = NULL;
 
 static float dts_delta_threshold = 10;
 
-static unsigned int sws_flags = SWS_BICUBIC;
-
 static int64_t timer_start;
 
 static uint8_t *audio_buf;