[libFuzzer] [Windows] Include windows.h and psapi.h with lowercase

This fixes building on a case sensitive filesystem with mingw-w64
headers, where all headers are lowercase, and matches how these
headers are included elsewhere in compiler-rt.

Also include these headers with angle brackets, as they are system
headers.

Differential Revision: https://reviews.llvm.org/D51913

llvm-svn: 341983