Fix #135606: Grease Pencil: Gesture selection not deselcting when clicking in empty space

Points are actually deselected, but the viewport is not updated.
Set `changed` variable to `true` to send update calls.

Pull Request: https://projects.blender.org/blender/blender/pulls/135708

source/blender/editors/grease_pencil/intern/grease_pencil_select.cc

@@ -273,6 +273,7 @@ bool selection_update(const ViewContext *vc,
             const IndexMask not_in_mask = changed_element_mask.complement(
                 selection.span.index_range(), memory);
             ed::curves::fill_selection_false(selection.span, not_in_mask);
+            changed = true;
             selection.finish();
           }
         }