x86_64: uml: fix rename header guard

In unistd_64.h, the guard macro _ASM_X86_64_UNISTD_H_ is renamed to
ASM_X86__UNISTD_64_H.

This change should be applied to arch/um/sys-x86_64/syscall_table.c.

Signed-off-by: Hiroshi Shimamoto <h-shimamoto@ct.jp.nec.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>