opengl: fixed compilation warning

modules/video_output/opengl.c

@@ -664,7 +664,7 @@ int vout_display_opengl_Prepare(vout_display_opengl_t *vgl,
         glBindTexture(vgl->tex_target, vgl->texture[0][j]);
 
 #ifndef GL_UNPACK_ROW_LENGTH
-        if ( (picture->p[j].i_pitch / picture->p[j].i_pixel_pitch) !=
+        if ( (picture->p[j].i_pitch / picture->p[j].i_pixel_pitch) != (unsigned int)
              ( picture->format.i_visible_width * vgl->chroma->p[j].w.num / vgl->chroma->p[j].w.den ) )
         {
             uint8_t *new_plane = malloc( picture->format.i_visible_width * vgl->fmt.i_visible_height * vgl->chroma->p[j].h.num / (vgl->chroma->p[j].h.den *  vgl->chroma->p[j].w.den ) );