udev: add missing new-line in udevadm error

fprintf() does not add new-lines automatically like log_*() does. Add the
missing \n specified so "udevadm" invoked without arguments adds a newline
to:
    udevadm: missing or unknown command

src/udev/udevadm.c

@@ -134,7 +134,7 @@ int main(int argc, char *argv[]) {
                                 goto out;
                         }
 
-        fprintf(stderr, "%s: missing or unknown command", program_invocation_short_name);
+        fprintf(stderr, "%s: missing or unknown command\n", program_invocation_short_name);
         rc = 2;
 out:
         label_finish();