[gnome-control-center] printers: Fix compile warning

[Georges Basile Stavracas Neto <gbsneto src gnome org>]

commit 2105e27bde1b453f652618a43e45496341f1dd8e
Author: Robert Ancell <robert ancell canonical com>
Date:   Thu Feb 7 09:44:14 2019 +1300

    printers: Fix compile warning
    
    Missing GTK_LABEL cast.

 panels/printers/pp-options-dialog.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)
---
diff --git a/panels/printers/pp-options-dialog.c b/panels/printers/pp-options-dialog.c
index 7bc32920f..a6747cfe2 100644
--- a/panels/printers/pp-options-dialog.c
+++ b/panels/printers/pp-options-dialog.c
@@ -350,7 +350,7 @@ ipp_option_add (IPPAttribute *attr_supported,
       position = grid_get_height (grid);
 
       label = gtk_label_new (option_display_name);
-      gtk_label_set_mnemonic_widget (label, widget);
+      gtk_label_set_mnemonic_widget (GTK_LABEL (label), widget);
       context = gtk_widget_get_style_context (label);
       gtk_style_context_add_class (context, "dim-label");
       gtk_widget_set_halign (label, GTK_ALIGN_END);
@@ -383,7 +383,7 @@ ppd_option_add (ppd_option_t  option,
       position = grid_get_height (grid);
 
       label = gtk_label_new (ppd_option_name_translate (&option));
-      gtk_label_set_mnemonic_widget (label, widget);
+      gtk_label_set_mnemonic_widget (GTK_LABEL (label), widget);
       context = gtk_widget_get_style_context (label);
       gtk_style_context_add_class (context, "dim-label");
       gtk_widget_set_halign (label, GTK_ALIGN_END);