fix invalid conversion error with MinGW 4.8.1 in net.cpp

- fixes src\net.cpp:1601: Error:invalid conversion from 'void*' to
  'const char*' [-fpermissive] in a setsockopt() call on Win32 that was
  found by using MinGW 4.8.1 compiler suite

See http://msdn.microsoft.com/en-us/library/windows/desktop/ms740476%28v=vs.85%29.aspx to verify that Win32 needs a ``const char *``.