vp8 realtime encoder: mark unused variable

'duration' is not used in realtime-only mode:

Cleans up -Wextra warning:
unused parameter 'duration' [-Wunused-parameter]

Change-Id: I827dfe59ebcdc72c5a93fdf7e5aca063433914b1

vp8/vp8_cx_iface.c

@@ -783,6 +783,7 @@ static void pick_quickcompress_mode(vpx_codec_alg_priv_t  *ctx,
     }
 
 #else
+    (void)duration;
     new_qc = MODE_REALTIME;
 #endif