msvc: Fix compilation errors caused by poll.h emulation

msvc: Fix compilation errors caused by poll.h emulation

Commit 0f77dea9 ("mingw: move poll out of sys-folder", 24-10-2011), along
with other commits in the 'ef/mingw-upload-archive' branch (see commit
7406aa20), effectively reintroduced the same problem addressed by commit
56fb3ddc ("msvc: Fix compilation errors in compat/win32/sys/poll.c",
04-12-2010).

In order to fix the compilation errors, we use the same solution adopted
in that earlier commit. In particular, we set _WIN32_WINNT to 0x0502
(which would target Windows Server 2003) prior to including the winsock2.h
header file.

Also, we delete the compat/vcbuild/include/sys/poll.h header file, since
it is now redundant and it's presence may cause some confusion.

Signed-off-by: Ramsay Jones <ramsay@ramsay1.demon.co.uk>
Tested-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>