mpm_winnt: Do not redefine the standard CONTAINING_RECORD() macro

mpm_winnt: Do not redefine the standard CONTAINING_RECORD() macro
in child.c.

This definition has been added in https://svn.apache.org/r88498 — perhaps,
because not every versions of SDK contained it at that time.

But since then, the macro has been available starting from Windows 2000
(https://msdn.microsoft.com/en-us/library/windows/hardware/ff542043),
and any available version of Windows SDK now should also contain it.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1801753 13f79535-47bb-0310-9956-ffa450edef68