libostree: Add ostree_sysroot_deployment_set_kargs()

It turns out people sometimes want to be able to change the kernel
arguments.  Add a convenient API to do so for the current deployment.

This will be used by Anaconda.

src/libostree/ostree-sysroot.c

@@ -1029,4 +1029,3 @@ ostree_sysroot_origin_new_from_refspec (OstreeSysroot  *sysroot,
   g_key_file_set_string (ret, "origin", "refspec", refspec);
   return ret;
 }
-