[glibmm] Gio::DBus::Introspection: Add a necessary #include

[Kjell Ahlstedt <kjellahl src gnome org>]

commit 283d820c2655a4333e3ca1167fb72c522b1f6dc2
Author: Kjell Ahlstedt <kjellahlstedt gmail com>
Date:   Wed Sep 18 11:04:05 2019 +0200

    Gio::DBus::Introspection: Add a necessary #include

 gio/src/dbusintrospection.hg | 1 +
 1 file changed, 1 insertion(+)
---
diff --git a/gio/src/dbusintrospection.hg b/gio/src/dbusintrospection.hg
index 367ccc98..f1ba9b55 100644
--- a/gio/src/dbusintrospection.hg
+++ b/gio/src/dbusintrospection.hg
@@ -15,6 +15,7 @@
  */
 
 #include <glibmm/refptr.h>
+#include <glibmm/value.h>
 #include <gio/gio.h>
 #include <vector>