x86: avoid name conflict for Voyager leave_mm

Avoid a conflict between Voyager's leave_mm and asm-x86/mmu.h's leave_mm.

Signed-off-by: Jeremy Fitzhardinge <jeremy@xensource.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>