unbound: fix getaddrinfo detection for 32-bit windows

On Windows, getaddrinfo is part of the Windows API and as such is
__stdcall, not __cdecl, so check_function_exists fails because the
declaration doesn't match the mangling __stdcall has. Instead, use a
header to include the symbol as declared on the system and use
check_symbol_exists instead.

Tested-By: greatwolf on IRC