Use WSAStartup()/WSACleanup() on WIN32

The alternative is to use these in every WIN32 application which uses xcb. Doing
it this way should be safe, as, according to MSDN, "There must be a call to
WSACleanup for each successful call to WSAStartup. Only the final WSACleanup
function call performs the actual cleanup. The preceding calls simply decrement
an internal reference count"

(We should probably also include ws2_32 in Libs.private for libxcb, as anything
which links with libxcb will also need that, but there seems to be some pkg-config
issues to resolve first...)

v2: Check for errors so WSAStartup()/WSACleanup() uses are balanced
v3: Use same indentation style as surrounding code

Reviewed-by: Peter Harris <pharris@opentext.com>
Signed-off-by: Julien Danjou <julien@danjou.info>