[libpeas] demo: add missing locale.h include

[Christian Hergert <chergert src gnome org>]

commit 11fe756da542c79c4a770babfaca0a99f22d6edb
Author: Christian Hergert <chergert redhat com>
Date:   Fri Mar 13 10:32:21 2020 -0700

    demo: add missing locale.h include
    
    Fixes #39

 peas-demo/peas-demo.c | 1 +
 1 file changed, 1 insertion(+)
---
diff --git a/peas-demo/peas-demo.c b/peas-demo/peas-demo.c
index 47ac005..466a392 100644
--- a/peas-demo/peas-demo.c
+++ b/peas-demo/peas-demo.c
@@ -26,6 +26,7 @@
 #include <girepository.h>
 #include <glib/gi18n.h>
 #include <gtk/gtk.h>
+#include <locale.h>
 
 #include <libpeas/peas.h>
 #include <libpeas-gtk/peas-gtk.h>