libcanberra: fix installed-vs-shipped warning

When build libcanberra for powerpc64 bsp, it shows warning:

WARNING: QA Issue: libcanberra: Files/directories were installed but not shipped
  /lib
  /lib/systemd
  /lib/systemd/system
  /lib/systemd/system/canberra-system-shutdown.service
  /lib/systemd/system/canberra-system-shutdown-reboot.service
  /lib/systemd/system/canberra-system-bootup.service [installed-vs-shipped]

Update FILES_${PN}-systemd to fix it.

Signed-off-by: Kai Kang <kai.kang@windriver.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>