[gimp] app: fix leaking a GimpMybrushSurface struct.

[Jehan <jehanp src gnome org>]

commit 24e44c13cd5a24e925f55711a2d30a5139b1c27d
Author: Jehan <jehan girinstud io>
Date:   Mon Aug 16 22:39:06 2021 +0200

    app: fix leaking a GimpMybrushSurface struct.
    
    Reported by Massimo, as again and again! \o/

 app/paint/gimpmybrushsurface.c | 1 +
 1 file changed, 1 insertion(+)
---
diff --git a/app/paint/gimpmybrushsurface.c b/app/paint/gimpmybrushsurface.c
index 9b4283cb61..436c77ced2 100644
--- a/app/paint/gimpmybrushsurface.c
+++ b/app/paint/gimpmybrushsurface.c
@@ -527,6 +527,7 @@ gimp_mypaint_surface_destroy (MyPaintSurface *base_surface)
 
   g_clear_object (&surface->buffer);
   g_clear_object (&surface->paint_mask);
+  g_free (surface);
 }
 
 GimpMybrushSurface *