[lldb] Fix TestFrameVarDILCast.py build on Windows AArch64
This patch adds <cstddef> to main.cpp in TestFrameVarDILCast.py so that
std::nullptr_t is properly declared. It fixes the TestFrameVarDILCast.py
compile failure observed on the LLDB Windows AArch64 buildbot:
https://lab.llvm.org/buildbot/#/builders/141
The issue was introduced by commit 539cf92 in #170332.
