repo: Add ostree_repo_write_symlink

Continuation of the addition of `ostree_repo_write_regfile_inline()`.
This will be helpful for ostree-rs-ext and importing from tar, it's
quite inefficient and awkward for small files to end up creating
a whole `GInputStream` and `GFileInfo` and etc. for small files.

src/libostree/ostree-repo.h

@@ -435,6 +435,16 @@ char *      ostree_repo_write_regfile_inline (OstreeRepo       *self,
                                                 GCancellable     *cancellable,
                                                 GError          **error);
 
+_OSTREE_PUBLIC
+char *      ostree_repo_write_symlink (OstreeRepo       *self,
+                                       const char       *expected_checksum,
+                                       guint32           uid,
+                                       guint32           gid,
+                                       GVariant         *xattrs,
+                                       const char       *symlink_target,
+                                       GCancellable     *cancellable,
+                                       GError          **error);
+
 _OSTREE_PUBLIC
 gboolean      ostree_repo_write_metadata_trusted (OstreeRepo        *self,
                                                   OstreeObjectType   objtype,