[LLDB] Rework a MinGW build fix from D65691

That change didn't contain any explanation for this bit. There shouldn't
be any need for a check for MinGW ifdefs here, as long as the include
uses lowercase windows.h (as is used consistently elsewhere in
the llvm projects).

Differential Revision: https://reviews.llvm.org/D67894

llvm-svn: 372656