Fix usage in Windows XP targetting projects

This is important since this stuff is in publicly included headers
and even though the usage of the WSAPoll API is a runtime decision
the public headers may be used in code that needs to build with
_WIN32_WINNT=0x0501 to support WinXP.

When building a project using libwebsockets with that define set
to 0x0501 winsock.h will not define the WSAPOLLFD struct causing that
project to fail to compile.