video_filter: motionblur: use C99 loop declaration

modules/video_filter/motionblur.c

@@ -174,11 +174,10 @@ static picture_t *Filter( filter_t *p_filter, picture_t *p_pic )
 static void RenderBlur( filter_sys_t *p_sys, picture_t *p_newpic,
                         picture_t *p_outpic )
 {
-    int i_plane;
     const int i_oldfactor = atomic_load( &p_sys->i_factor );
     int i_newfactor = 128 - i_oldfactor;
 
-    for( i_plane = 0; i_plane < p_outpic->i_planes; i_plane++ )
+    for( int i_plane = 0; i_plane < p_outpic->i_planes; i_plane++ )
     {
         uint8_t *p_old, *p_new, *p_out, *p_out_end, *p_out_line_end;
         const int i_visible_pitch = p_outpic->p[i_plane].i_visible_pitch;