[rhythmbox/wip/sam/meson] Fix warning that breaks CI

[Sam Thursfield <sthursfield src gnome org>]

commit 9a991aaa315b7747876c911712fe17848b6db8ab
Author: Sam Thursfield <sam afuera me uk>
Date:   Tue Oct 27 19:51:58 2020 +0100

    Fix warning that breaks CI
    
    Perhaps a dependency updated as I don't see this in master, but I do see at
    https://gitlab.gnome.org/GNOME/rhythmbox/-/jobs/960290
    
        ../../metadata/rb-ext-db.c: In function 'impl_constructor':
        /usr/include/glib-2.0/gobject/gobject.h:512:33: error: return from incompatible pointer type 
[-Werror=incompatible-pointer-types]
         #define g_object_ref(Obj)      ((__typeof__(Obj)) (g_object_ref) (Obj))
                                        ~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
        ../../metadata/rb-ext-db.c:349:11: note: in expansion of macro 'g_object_ref'
            return g_object_ref (inst);
                   ^~~~~~~~~~~~
        cc1: all warnings being treated as errors

 metadata/rb-ext-db.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/metadata/rb-ext-db.c b/metadata/rb-ext-db.c
index 2307bca60..ec9e3f613 100644
--- a/metadata/rb-ext-db.c
+++ b/metadata/rb-ext-db.c
@@ -346,7 +346,7 @@ impl_constructor (GType type, guint n_construct_properties, GObjectConstructPara
                RBExtDB *inst = l->data;
                if (g_strcmp0 (name, inst->priv->name) == 0) {
                        rb_debug ("found existing metadata store %s", name);
-                       return g_object_ref (inst);
+                       return (GObject *) g_object_ref (inst);
                }
        }