local-display-factory: fix build with --disable-user-display-server

commit cf8d918f added a call to `lookup_by_session` which
was previously only defined when built GDM was built with
`--enable-user-disable-server`.

That causes the build to fail with `--disable-user-display-server`.

This commit fixes the build by moving `lookup_by_session` out
of conditionalized code.

Closes #416
1 job for <span class="ref-name">BUILD-FIXES-EEK</span> in 3 minutes and 41 seconds
Status Job ID Name Coverage
  Build
passed #95044
build-fedora

00:03:41