libapparmor: add missing include for `socklen_t`

Merged Patrick Steinhardt requested to merge (removed):pks-missing-include into master

While include/sys/apparmor.h makes use of socklen_t, it doesn't include the <sys/socket.h> header to make its declaration available. While this works on systems using glibc via transitive includes, it breaks compilation on musl libc.

Fix the issue by including the header.