Fix Windows build by including wstcpip.h before winsock.h is included

Error generated by socket-io.c was:

/usr/i686-pc-mingw32/sys-root/mingw/include/ws2tcpip.h:38:2:
error: #error "ws2tcpip.h is not compatible with winsock.h.
Include winsock2.h instead."

Signed-off-by: Alex J Lennon <ajlennon@dynamicdevices.co.uk>
[CI] Add appveyor.yml configuration file to support Windows build

Adds a configuration file to facilitate build of Mono with Cygwin
on Appveyor Windows-based build workers.

Dependencies are installed on a clean Appveyor build image for
each build and the result of the 'make install' step is stored
as a build output 'artifact' for further testing or deployment.

Signed-off-by: Alex J Lennon <ajlennon@dynamicdevices.co.uk>