[balsa] cell-renderer-button: Fix the build

[Peter Bloomfield <peterb src gnome org>]

commit 313d154f1e98a3ad44c0442b6e891f869ab56eb7
Author: Peter Bloomfield <PeterBloomfield bellsouth net>
Date:   Tue Jun 4 13:15:41 2019 -0400

    cell-renderer-button: Fix the build
    
    Restore the struct definition, and actually make it private.

 libbalsa/cell-renderer-button.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)
---
diff --git a/libbalsa/cell-renderer-button.c b/libbalsa/cell-renderer-button.c
index 6a95cd6ee..831604385 100644
--- a/libbalsa/cell-renderer-button.c
+++ b/libbalsa/cell-renderer-button.c
@@ -30,11 +30,15 @@ enum {
     LAST_SIGNAL
 };
 
-static guint cell_button_signals[LAST_SIGNAL] = { 0 };
+struct _LibBalsaCellRendererButton {
+    GtkCellRendererPixbuf parent;
+};
 
 G_DEFINE_TYPE(LibBalsaCellRendererButton, libbalsa_cell_renderer_button,
               GTK_TYPE_CELL_RENDERER_PIXBUF)
 
+static guint cell_button_signals[LAST_SIGNAL] = { 0 };
+
 static gboolean
 libbalsa_cell_renderer_button_activate(GtkCellRenderer    * cell,
                                        GdkEvent           * event,