x86: hide cond_syscall behind __KERNEL__

This brings x86_64 into line with all other architectures by only defining
cond_syscall() when __KERNEL__ is defined.

[ tglx: arch/x86 adaptation ]

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Andi Kleen <ak@suse.de>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>