ucred.h: Do not include audit.h unconditionally

[markj]

Summary

audit.h is included because struct ucred embeds a struct auditinfo_addr,
which is defined in audit.h. But struct ucred is not visible to
userspace by default, so we shouldn't be including audit.h by default
either, especially since audit.h pulls in param.h, which pollutes the
namespace. (In particular it pulls in machine/param.h, which #defines
"MACHINE", which makes the QEMU build unhappy.)

MFC after: 1 week

[markj]

Based on the amount of userspace code I've had to fix in the base system after this patch, we'll need to do an exp-run.