win32: fix Visual Studio 2010 build with WINVER >= 0x600

If no WINVER and/or _WIN32_IWNNT define was set, the Windows platform
SDK often defaults to high value, e.g. 0x601 (whoch may probably depend
on the Windows version being used, in my case Windows 7).

If WINVER >= 0x600 then winsock2.h includes some defines for WSAPoll(),
e.g. POLLIN, POLLPRI, POLLOUT etc. These defines clash with cURL's
lib/select.h.

Make sure HAVE_STRUCT_POLLFD is defined then.

Bug: http://curl.haxx.se/bug/view.cgi?id=1282
Reported-by: "kdekker"
Patch-by: Marcel Raad