powermand: fix malformed conditional

Problem: a conditional block was not enclosed in braces.

Clang found this one in CI:

client.c: In function ‘cli_pre_poll’:
client.c:1070:9: error: this ‘if’ clause does not guard... [-Werror=misleading-indentation]
 1070 |         if (listen_fds[i] != NO_FD)
      |         ^~
client.c:1072:13: note: ...this statement, but the latter is misleadingly indented as if it were guarded by the ‘if’
 1072 |             xpollfd_set(pfd, listen_fds[i], XPOLLIN);
      |             ^~~~~~~~~~~

powermand/client.c

@@ -1067,9 +1067,10 @@ void cli_pre_poll(xpollfd_t pfd)
     int i;
 
     for (i = 0; i < listen_fds_len; i++) {
-        if (listen_fds[i] != NO_FD)
+        if (listen_fds[i] != NO_FD) {
             assert(listen_fds[i] >= 0);
             xpollfd_set(pfd, listen_fds[i], XPOLLIN);
+        }
     }
 
     itr = list_iterator_create(cli_clients);