Bump debhelper from old 10 to 12.

+ debian/rules: Move --fail-missing argument to dh_missing.

Fixes lintian: package-uses-old-debhelper-compat-version
See https://lintian.debian.org/tags/package-uses-old-debhelper-compat-version.html for more details.

debian/compat

@@ -1 +1 @@
-10
+12

debian/control

@@ -5,7 +5,7 @@ Maintainer: Live Systems Maintainers <debian-live@lists.debian.org>
 Uploaders: Iain R. Learmonth <irl@debian.org>, Raphaël Hertzog <hertzog@debian.org>,
  Luca Boccassi <bluca@debian.org>
 Build-Depends:
- debhelper (>= 10),
+ debhelper (>= 12~),
  po4a,
 Standards-Version: 4.4.1
 Rules-Requires-Root: no

debian/rules

@@ -13,5 +13,5 @@ override_dh_auto_install:
 	mkdir -p debian/live-boot-initramfs-tools/usr/share
 	mv debian/tmp/usr/share/initramfs-tools debian/live-boot-initramfs-tools/usr/share
 
-override_dh_install:
-	dh_install --fail-missing
+override_dh_missing:
+	dh_missing --fail-missing