Use HAVE_SECCOMP_NOTIFY instead of HAVE_DECL_SECCOMP_NOTIFY_FD.
Currently the latter will be true if the declaration is found by
configure, even if 'configure --disable-seccomp' is used.

HAVE_SECCOMP_NOTIFY is defined in lxcseccomp.h if both HAVE_SECCOMP and
HAVE_DECL_SECCOMP_NOTIFY_FD are true, which is the correct behavior.
Signed-off-by: Eneas U de Queiroz <cotequeiroz@gmail.com>