Allow (with a license warning) "options ZFS" to work in static kernels.

The 'make depend' rules have to use custom -I paths for the special compat
includes for the opensolaris/zfs headers.

This option will pull in the couple of files that are shared with dtrace,
but they appear to correctly use the MODULE_VERSION/MODULE_DEPEND rules
so loader should do the right thing, as should kldload.

Reviewed by: pjd (glanced at)