Add an API to set/unset a deployment tree's mutability

This will be used by rpm-ostree to unset the immutable bit temporarily
in order to do package layering.  We could add an API to deploy a tree
without the immutable bit, but this is simpler.

src/libostree/ostree-sysroot.h

@@ -96,6 +96,12 @@ gboolean ostree_sysroot_deploy_tree (OstreeSysroot     *self,
                                      GCancellable      *cancellable,
                                      GError           **error);
 
+gboolean ostree_sysroot_deployment_set_mutable (OstreeSysroot     *self,
+                                                OstreeDeployment  *deployment,
+                                                gboolean           mutable,
+                                                GCancellable      *cancellable,
+                                                GError           **error);
+
 OstreeDeployment *ostree_sysroot_get_merge_deployment (OstreeSysroot     *self,
                                                        const char        *osname);