regression tests: Don't build syscall_sysctl if missing kernel headers

sys/sysctl.h is not guaranteed to exist anymore since https://sourceware.org/pipermail/glibc-cvs/2020q2/069366.html

which is a follow on to the kernel commit 61a47c1ad3a4 sysctl: Remove the sysctl system call

While the syscall_sysctl currently checks if the kernel supports sysctrs before running the tests. The tests can't even build if the kernel headers don't have the sysctl defines.

Fixes: #119 (closed) Fixes: https://bugs.launchpad.net/apparmor/+bug/1897288 Signed-off-by: John Johansen john.johansen@canonical.com