Including <winsock.h> and <winsock2.h> is incompatible and if the
application wants to use the latter, it may be convenient to define
wxUSE_WINSOCK2 when building wxWidgets instead of having to work around
winsock.h implicit inclusion from include/wx/msw/wrapwin.h.
Closes https://github.com/wxWidgets/wxWidgets/pull/1122