peas-demo: missing header for setlocale()

Please include <locale.h> for setlocale() in peas-demo.c:

--- peas-demo/peas-demo.c.orig  2020-03-06 21:12:16.247286600 +0000
+++ peas-demo/peas-demo.c
@@ -30,6 +30,8 @@
 #include <libpeas/peas.h>
 #include <libpeas-gtk/peas-gtk.h>
 
+#include <locale.h>
+
 #include "peas-demo-window.h"
 
 gboolean run_from_build_dir;

This fixes compilation on NetBSD/amd64.