Add missing G_BEGIN/END_DECLS to gsettingsschema.h.

Without this fix, using any of the enclosed functions when building with a
C++ compiler will result in undefined references.

https://bugzilla.gnome.org/show_bug.cgi?id=703478

gio/gsettingsschema.h

@@ -23,6 +23,8 @@
 
 #include <glib-object.h>
 
+G_BEGIN_DECLS
+
 typedef struct _GSettingsSchemaSource                       GSettingsSchemaSource;
 typedef struct _GSettingsSchema                             GSettingsSchema;
 
@@ -62,4 +64,6 @@ const gchar *           g_settings_schema_get_id                        (GSettin
 GLIB_AVAILABLE_IN_2_32
 const gchar *           g_settings_schema_get_path                      (GSettingsSchema        *schema);
 
+G_END_DECLS
+
 #endif /* __G_SETTINGS_SCHEMA_H__ */