sysroot: Add _require_booted_deployment() API

This is a common pattern that is replicated both in our code
and in rpm-ostree a lot.  Let's add a canonical API.

src/libostree/libostree-devel.sym

@@ -25,6 +25,7 @@
 LIBOSTREE_2021.1 {
 global:
   ostree_commit_metadata_for_bootable;
+  ostree_sysroot_require_booted_deployment;
 } LIBOSTREE_2020.8;
 
 /* Stub section for the stable release *after* this development one; don't