rtmp: fix for compiling with lwIP

Compiling on _WIN32 and with USE_LWIPSOCK, causes this error:
  curl_rtmp.c(223,3):  error: use of undeclared identifier 'setsockopt'
    setsockopt(r->m_sb.sb_socket, SOL_SOCKET, SO_RCVTIMEO,
    ^
  curl_rtmp.c(41,32):  note: expanded from macro 'setsockopt'
  #define setsockopt(a,b,c,d,e) (setsockopt)(a,b,c,(const char *)d,(int)e)
                                 ^
Closes #3155