pseudo: add macro guard for seccomp

It fails to compile pseudo-native on centos 7:

| ports/linux/pseudo_wrappers.c: In function ‘prctl’:
| ports/linux/pseudo_wrappers.c:129:14: error: ‘SECCOMP_SET_MODE_FILTER’ undeclared (first use in this function)
|    if (cmd == SECCOMP_SET_MODE_FILTER) {
|               ^

Add macro guard for the definition to avoid the failure.

Signed-off-by: Kai Kang <kai.kang@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
Signed-off-by: Steve Sakoman <steve@sakoman.com>