nspawn: add -M option to optstring

This was missed in commit 7027ff61a3 and
means that the --machine option would work but not its shorthand, -M.

src/nspawn/nspawn.c

@@ -173,7 +173,7 @@ static int parse_argv(int argc, char *argv[]) {
         assert(argc >= 0);
         assert(argv);
 
-        while ((c = getopt_long(argc, argv, "+hD:u:C:bj", options, NULL)) >= 0) {
+        while ((c = getopt_long(argc, argv, "+hD:u:C:bM:j", options, NULL)) >= 0) {
 
                 switch (c) {