x86-64: Don't export init_level4_pgt

It's not used by any module, and i386 (as well as some other arches)
also doesn't export its equivalent (swapper_pg_dir).

Signed-off-by: Jan Beulich <jbeulich@novell.com>
LKML-Reference: <4BCF33BD020000780003B3E4@vpn.id2.novell.com>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>