vsyscall_64: add missing ifdef CONFIG_SECCOMP

vsyscall_seccomp introduced a dependency on __secure_computing.  On
configurations with CONFIG_SECCOMP disabled, compilation will fail.

Reported-by: feng xiangjun <fengxj325@gmail.com>
Signed-off-by: Will Drewry <wad@chromium.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>