msvc: Fix compilation errors in compat/win32/sys/poll.c

msvc: Fix compilation errors in compat/win32/sys/poll.c

The msvc winsock2.h header file conditionally defines or declares
poll() related symbols which cause many macro redefinition errors,
a struct type redefinition error and syntax errors. These symbols
are defined in support of the WSAPoll() API, new in Windows Vista,
when the symbol _WIN32_WINNT is defined and _WIN32_WINNT >= 0x0600.

In order to avoid the compilation errors, we set _WIN32_WINNT to
0x0502 (which would target Windows Server 2003) prior to including
the winsock2.h header file.

Signed-off-by: Ramsay Jones <ramsay@ramsay1.demon.co.uk>
Acked-by: Erik Faye-Lund <kusmabite@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>