Include dmsetup in the core18 snap

The dmsetup package includes udev rules that are essential for any system
using device mapper, which includes any system using crypsetup and we want
to support full-disk encryption in Ubuntu Core.

Bug-Ubuntu: https://bugs.launchpad.net/bugs/1835846

debian/changelog

@@ -1,3 +1,9 @@
+livecd-rootfs (2.408.51) UNRELEASED; urgency=medium
+
+  * Add dmsetup to the core and core16 snaps.  LP: #1835846.
+
+ -- Steve Langasek <steve.langasek@ubuntu.com>  Mon, 08 Jul 2019 16:39:24 -0700
+
 livecd-rootfs (2.408.50) xenial; urgency=medium
 
   * Actually, do not depend on snapd on powerpc as well.  Snaps are not

live-build/auto/config

@@ -467,6 +467,7 @@ case $PROJECT in
 			# no Task: header yet
 			add_package install snapd
 			add_package install ubuntu-core-snapd-units
+			add_package install dmsetup
 
 			# no minimal as we want to be really minimal
 			#add_task install minimal