libzfs_core provides a rather limited but committed (stable) interface
for working with ZFS. We install libzfs_core shared library but we do
not install header files required for developing programs that use
the library. This change is to install the required header files
libzfs_core.h, libnvpair.h and sys/nvpair.h.

As this is the first time I am touching include/Makefile, I am not sure
if my change is correct.

In this change I suggest installing the headers into the same locations
as on illumos.

Another alternative is to follow what ZoL does and install the headers
under /usr/include/libzfs directory.
Yet another possibility is to install libnvpair.h and sys/nvpair into
a directory like /usr/include/illumos, because those headers are not
specific to ZFS. libzfs_core.h could be installed either at the top
level or under libzfs/ directory.