libzfs: Add missing files into libzfs build to fix missing symbols

Some functionality from the zfs and zpool commands require some functions
contained in zfs_prop.c and zpool_prop.c. This patch solves this problem
by adding the files as part of the libzfs compilation process.
Currently, OSv fall into the 'failed to lookup symbol' scenario when using
certain parameters from the aforementioned commands, e.g.:
scripts/run.py -e 'zfs.so list osv'

_OPENSOLARIS_SYS_UIO_H_ was added to CFLAGS to fix a confliction where
'struct uio' was being defined twice (Glauber Costa).

Reviewed-by: Glauber Costa <glommer@cloudius-systems.com>
Signed-off-by: Raphael S. Carvalho <raphaelsc@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>