Fix (unreported) GPencil cannot deselect points with box/lasso select

Selecting in empty space wasnt considered as 'changed'.

Differential Revision: https://developer.blender.org/D8770

source/blender/editors/gpencil/gpencil_select.c

@@ -1114,6 +1114,8 @@ static int gpencil_generic_select_exec(
       gps->flag &= ~GP_STROKE_SELECT;
     }
     CTX_DATA_END;
+
+    changed = true;
   }
 
   /* select/deselect points */