x86: compile apm and voyager module only when selected in Kconfig

Previously the complete files were #ifdef'ed, but now handle that in the
Makefile.

May save a minor bit of compilation time.

[ Stephen Rothwell <sfr@canb.auug.org.au>: build dependency fix ]
Signed-off-by: Andi Kleen <ak@suse.de>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>