x86_64: asm/ptrace.h needs linux/compiler.h

On x86_64, <asm/ptrace.h> uses __user but doesn't include
<linux/compiler.h>.  This could lead to build failures.

Signed-off-by: Jean Delvare <khali@linux-fr.org>
Signed-off-by: Andi Kleen <ak@suse.de>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>