Install header during post-build rather than post-install.

New versions of dkms clean up the build directory after installing.

It appears that this was always intended, but had rm -rf "/path/to/build/*"
(note the quotes), which prevented it from working.

Also, the build step is already installing stuff into the directory where
these files go, so installing our stuff there as part of build rather than
install makes sense.

Signed-off-by: Tom Prince <tom.prince@clusterhq.com>
Signed-off-by: Richard Yao <ryao@gentoo.org>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #399