mapi32: Replace CRITICAL_SECTION by RTL_CRITICAL_SECTION.

[Robert Shearman]

Hervé Poussineau wrote:

>Replace CRITICAL_SECTION by RTL_CRITICAL_SECTION, according to prototype of
>RtlEnterCriticalSection and others.
>

It would be better to fix the file to use the kernel32 critical section 
functions instead of the ntdll ones.

-- 
Rob Shearman