meson: don't install broken tmpfiles config with sshd?confdir == 'no'

20-systemd-ssh-generator.conf expands SSHCONFDIR, which is bogus when we
build with -Dsshconfdir=no. Similarly, avoid expanding SSHDCONFDIR in
20-systemd-userdb.conf when building with -Dsshconfdir=no.

Follow-up 6c7fc5d5f2.

src/ssh-generator/meson.build

@@ -11,7 +11,7 @@ executables += [
         },
 ]
 
-if sshconfdir != 'no'
+if conf.get('ENABLE_SSH_PROXY_CONFIG') == 1
         custom_target(
                 '20-systemd-ssh-proxy.conf',
                 input : '20-systemd-ssh-proxy.conf.in',