syscalls/prctl.c: Fix build with USE_SECCOMP disabled.

This patch fixes this build error on CentOS 6.

  CC	syscalls/prctl.o
syscalls/prctl.c:48: error: expected ‘)’ before ‘int’
cc1: warnings being treated as errors
syscalls/prctl.c: In function ‘sanitise_prctl’:
syscalls/prctl.c:63: error: implicit declaration of function ‘do_set_seccomp’
syscalls/prctl.c:63: error: nested extern declaration of ‘do_set_seccomp’

Signed-off-by: Vinson Lee <vlee@twitter.com>