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/ostree-sysroot.h

@@ -85,6 +85,9 @@ GPtrArray *ostree_sysroot_get_deployments (OstreeSysroot  *self);
 _OSTREE_PUBLIC
 OstreeDeployment *ostree_sysroot_get_booted_deployment (OstreeSysroot *self);
 _OSTREE_PUBLIC
+OstreeDeployment *
+ostree_sysroot_require_booted_deployment (OstreeSysroot *self, GError **error);
+_OSTREE_PUBLIC
 OstreeDeployment *ostree_sysroot_get_staged_deployment (OstreeSysroot *self);
 
 _OSTREE_PUBLIC