dbus: fix dbus symlink removal

There was already a post-build hook to delete the /var/lib/dbus symlink
created by buildroot after the package's own installation, to prevent
a dbus installation error during "make dbus-rebuild".  However, this
misses the case for when one might delete the .stamp_target_installed
file manually, outside of dbus-rebuild.  This can be fixed by changing
the post-build hook to a pre-install hook.  This seems appropriate,
since it is really addressing an installation issue, not a build issue.

Signed-off-by: Danomi Manchego <danomimanchego123@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>