Add missing libzfs_core to Makefiles

On some platforms symbols provided by libzfs_core and used by
libzfs were not available to the linker. To avoid this issue
libzfs_core has been added to the list of required libraries
when building utilities which depend on libzfs. This should
have been handled properly by libtool and it's still not
entirely clear why it wasn't on all platforms.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #1841