build: Fix some harmless compiler warnings

Some (newer?) GCC versions complain when a g_auto variable isn't
initialized when declared, even when the initialization is guaranteed
to happen before the variable is used or goes out of scope.

https://gitlab.gnome.org/GNOME/gnome-shell/issues/2298

src/st/st-texture-cache.c

@@ -142,7 +142,7 @@ on_icon_theme_changed (StSettings     *settings,
                        GParamSpec     *pspec,
                        StTextureCache *cache)
 {
-  g_autofree gchar *theme;
+  g_autofree gchar *theme = NULL;
 
   st_texture_cache_evict_icons (cache);