Fix #124990: GP Draw mode always adds black color to a palette

Caused by fe0e2907b3

Prior to that commit, adding palette colors was only working because
`BKE_paintmode_get_active_from_context` was falling through to
`PaintMode::Texture2D` which then would do its thing in
`palette_color_add_exec`

To resolve, now respect `PaintMode::GPencil` as well there.

Pull Request: https://projects.blender.org/blender/blender/pulls/125062

source/blender/editors/sculpt_paint/paint_ops.cc

@@ -180,7 +180,8 @@ static int palette_color_add_exec(bContext *C, wmOperator * /*op*/)
              PaintMode::Texture3D,
              PaintMode::Texture2D,
              PaintMode::Vertex,
-             PaintMode::Sculpt))
+             PaintMode::Sculpt,
+             PaintMode::GPencil))
     {
       copy_v3_v3(color->rgb, BKE_brush_color_get(scene, brush));
       color->value = 0.0;