Install dtrace.h and dependencies

This makes the minimum amount of changes to allow inclusion of dtrace.h
without all the solaris compatibility headers. Installing dtrace.h allows
compiling consumers of libdtrace (e.g. https://github.com/tmetsch/python-dtrace)
without requiring a copy of the source tree.
For python-dtrace I worked around this in https://github.com/tmetsch/python-dtrace/commit/58019c9a12022203a9ffda286dd8b41f1a5ace42
but being able to build the library without installed sources would be
extremely useful.

Reviewed By: gnn
Differential Revision: https://reviews.freebsd.org/D27884