build: add new file, for lxc_protocol checking

Commit 509eb51e added lxc_protocol.x; but without the initial
checkin of src/lxc_protocol-structs, 'make check' would fail for
anyone with pdwtags installed:
make[3]: *** No rule to make target `lxc_protocol-structs', needed by `check-protocol'.  Stop.

* src/lxc_protocol-structs: New file.

src/lxc_protocol-structs

+/* -*- c -*- */
+struct remote_nonnull_domain {
+        remote_nonnull_string      name;
+        remote_uuid                uuid;
+        int                        id;
+};
+struct lxc_domain_open_namespace_args {
+        remote_nonnull_domain      dom;
+        u_int                      flags;
+};
+enum lxc_procedure {
+        LXC_PROC_DOMAIN_OPEN_NAMESPACE = 1,
+};