Fix -Werror build failure in RtsMain
Something in Excn.h's include chain is loading _mingw.h which is defining a macro that
PosixSource.h is going to define.
_mingw.h's version properly checks if it has already been defined and skips it, so fixing the warning can be done
by just including Excn.h later (moved it to before last include).
Test Plan: ./validate
Reviewed By: austin
Differential Revision: https://phabricator.haskell.org/D698