src/privsep-linux.c: add support for xtensa (#22)

 src/privsep-linux.c: add support for xtensa (#22)

Fix the following build failure:

privsep-linux.c:206:4: error: #error "Platform does not support seccomp filter yet"
 #  error "Platform does not support seccomp filter yet"
    ^~~~~
In file included from privsep-linux.c:36:
privsep-linux.c:213:38: error: 'SECCOMP_AUDIT_ARCH' undeclared here (not in a function); did you mean 'SECCOMP_ALLOW_ARG'?
  BPF_JUMP(BPF_JMP + BPF_JEQ + BPF_K, SECCOMP_AUDIT_ARCH, 1, 0),
                                      ^~~~~~~~~~~~~~~~~~

It should be noted that AUDIT_ARCH_XTENSA is only defined since kernel
5.0 and
https://github.com/torvalds/linux/commit/98c3115a4ec56f03056efd9295e0fcb4c5c57a85

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>