local.conf.sample.extended: Mention shadow-base for login_manager for systemd

While not strictly required, list shadow-base as the example to use for
VIRTUAL-RUNTIME_login_manager when configuring for systemd.  This
provider matches what is found in for example Debian and is also one of
the non-intuitive VIRTUAL-RUNTIME alternatives, so it's good to have an
example in use somewhere.

(From meta-yocto rev: 7a75d20f1361a64bebfd2ad228ebde1a823fd239)

Signed-off-by: Tom Rini <trini@konsulko.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta-poky/conf/local.conf.sample.extended

@@ -395,5 +395,6 @@
 #
 #DISTRO_FEATURES_append = " systemd"
 #DISTRO_FEATURES_BACKFILL_CONSIDERED += "sysvinit"
+#VIRTUAL-RUNTIME_login_manager = "shadow-base"
 #VIRTUAL-RUNTIME_init_manager = "systemd"
 #VIRTUAL-RUNTIME_initscripts = "systemd-compat-units"