Add dbus-x11 to install-build-deps

dbus_unittests relies on the presence of /usr/bin/dbus-launch, which is provided
by the dbus-x11 package.  The absence of this package is currently causing
dbus_unittests to fail on the Linux Xenial bot.

BUG=789768
R=dpranke
NOTRY=true

Change-Id: Ic844648f3651e518e276c1fb764849b09145b34b
Reviewed-on: https://chromium-review.googlesource.com/924693
Commit-Queue: Thomas Anderson <thomasanderson@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#537483}

build/install-build-deps.sh

@@ -150,6 +150,7 @@ dev_list="\
   bison
   cdbs
   curl
+  dbus-x11
   dpkg-dev
   elfutils
   devscripts