bugfix for the x64 pointer truncation issue (Fixes #8361)

Hi this should fix issue #8361 (Note I don't have a suitable meterpreter build environment at the moment so cant test, but I patched my ReflectiveDllInjection repo and tested the fix to work on that so this should be ok).

A define in ReflectiveDLLInjection.h was truncating large pointers ...facepalm :/

(Might be useful at some point to make the Meterpreter RDI stuff a git submodule to easily keep sync).