package/libseccomp: works only on x86

As it is, the libseccomp code explicitly checks for x86 (32- or 64-bit),
so it can't work on other architectures.

Reported-by: Peter Korsgaard <jacmet@uclibc.org>
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>