MinGW: use nameless unions in ext_ad_group_acl (#2004)

MinGW: use nameless unions in ext_ad_group_acl (#2004)

ext_ad_group_acl was written in 2008 in C, and
it used the C variant of the Win32 API.
It was then ported to C++, but the API callers were
not updated to the C++ version of the API.
With more modern compilers, and
Squid enforcing more strict types and error handling,
it is no longer compiling.

This is part 1 of 2 of the fixes to make the helper build
again, the scope is to update Win32 API callers so they
use the C++ version of the API

Examples of fixed errors:

    error: 'IADs' {aka 'struct IADs'} has no member named 'lpVtbl'
    error: 'VARIANT' {aka 'struct tagVARIANT'} has no member named 'n1'