stg/Prim.h: drop redundant #ifdef

Summary:
Noticed by Herbert Valerio Riedel

Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>

Test Plan: build test

Reviewers: simonmar, austin, hvr

Reviewed By: hvr

Subscribers: rwbarton, phaskell, simonmar, relrod, ezyang, carter

Differential Revision: https://phabricator.haskell.org/D143