Add libnss-mdns to list of installed packages.

libnss-mdns allows to resolve *.local domain names, e.g.
'ping aiy.local' works.

Change-Id: I38bfca24d6a0f93f1a333e21bf1d2d93c8be9534
diff --git a/preamble.mk b/preamble.mk
index b292c16..c8b39c4 100644
--- a/preamble.mk
+++ b/preamble.mk
@@ -54,6 +54,7 @@
 	libclass-isa-perl \
 	libinput10 \
 	liblockfile-bin \
+	libnss-mdns \
 	libpam-systemd \
 	libswitch-perl \
 	libwayland-cursor0 \