lib: Use g_file_enumerator_iterate() if available, with fallback

Import `gs_file_enumerator_iterate()` for the next six months or
so...after RHEL 7.3 is released I'm strongly considering hard
requiring 2.46 or so.

Likely at some point we should figure out how to share more "glib
backport" code with NetworkManager at least.

Closes: #341
Approved by: jlebon

src/libostree/ostree-bootloader-grub2.c

@@ -83,8 +83,8 @@ _ostree_bootloader_grub2_query (OstreeBootloader *bootloader,
           const char *fname;
           g_autofree char *subdir_grub_cfg = NULL;
 
-          if (!gs_file_enumerator_iterate (direnum, &file_info, NULL,
-                                           cancellable, error))
+          if (!g_file_enumerator_iterate (direnum, &file_info, NULL,
+                                          cancellable, error))
             goto out;
           if (file_info == NULL)
             break;