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/libostree-devel.sym

@@ -25,6 +25,7 @@
 LIBOSTREE_2021.2 {
 global:
   ostree_repo_write_regfile_inline;
+  ostree_repo_write_symlink;
 } LIBOSTREE_2021.1;
 
 /* Stub section for the stable release *after* this development one; don't