tcpconns plugin: fix implicit conversion warnings

  CC       src/tcpconns.lo
src/tcpconns.c:510:14: warning: implicit conversion loses integer precision: 'ssize_t' (aka 'long') to 'int' [-Wshorten-64-to-32]
    status = recvmsg(fd, (void *)&msg, /* flags = */ 0);
           ~ ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
src/tcpconns.c:577:20: warning: implicit conversion loses integer precision: 'unsigned long' to 'int' [-Wshorten-64-to-32]
  int buffer_len = strlen(buffer);
      ~~~~~~~~~~   ^~~~~~~~~~~~~~
2 warnings generated.