Don't ifdef KDTRACE_HOOKS struct, variable, and function prototype
definitions for the DTrace audit provider, so that the dtaudit module
can compile in the absence of kernel DTrace support. This doesn't
really make run-time sense (since the binary dependencies for the
module won't be present), but it allows the dtaudit module to compile
successfully regardless of the kernel configuration.

MFC after: 3 weeks
Sponsored by: DARPA, AFRL
Reported by: kib