x86: clean up vdso_enabled type on x86_64

This fixes type of "vdso_enabled" on X86_64 to match extern in asm/elf.h.

Signed-off-by: OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>