lvm2: enable lvmetad

the systemd unit file of lvm2-monitor requires lvm2-lvmetad.socket which
asks to enable lvmetad, lvmetad is a metadata caching daemon for LVM. so
this fixed the below error:
  # systemctl start lvm2-monitor
  Failed to start lvm2-monitor.service: Unit lvm2-lvmetad.socket failed to load: No such file or directory.
  #

replace the installation of system unit file by make install_systemd_units

Signed-off-by: Roy Li <rongqing.li@windriver.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>