policycoreutils: override DESTDIR instead of PREFIX

PREFIX should always point to $(DESTDIR)/usr in order to work in the SDK
where /lib64 isn't a symlink to /usr/lib64 like it is in our images.

sdk_container/src/third_party/coreos-overlay/sys-apps/policycoreutils/policycoreutils-2.4-r2.ebuild

@@ -109,7 +109,7 @@ src_compile() {
 			INOTIFYH="$(usex dbus)" \
 			SESANDBOX="n" \
 			CC="$(tc-getCC)" \
-			PREFIX="${ROOT}" \
+			DESTDIR="${ROOT}" \
 			PYLIBVER="${EPYTHON}" \
 			LIBDIR="\$(PREFIX)/$(get_libdir)"
 	}