Fix build error when make deb

After 53698a4, the following error occurs when make deb.

CCLD     zed

../../lib/libzfs/.libs/libzfs.so: undefined reference to `get_system_hostid'

Add libzpool.la to zed/Makefile.am to fix this

Signed-off-by: Chunwei Chen <tuxoko@gmail.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #3080