debian: Switch to debhelper compatibility level 9

debian/changelog

@@ -82,6 +82,7 @@ dpkg (1.18.0) UNRELEASED; urgency=low
   * Packaging:
     - Remove old trigger related Breaks and Conflicts from dpkg.
     - Only use stackprotectorstrong when building dpkg with gcc >= 4.9.
+    - Switch to debhelper compatibility level 9.
   * Documentation:
     - Document arch-qualifiers for dependency fields in deb-control(5) and
       deb-src-control(5). Reported by Johannes Schauer <j.schauer@email.de>.

debian/compat

@@ -1 +1 @@
-7
+9

debian/control

@@ -9,7 +9,7 @@ Homepage: https://wiki.debian.org/Teams/Dpkg
 Vcs-Browser: https://anonscm.debian.org/cgit/dpkg/dpkg.git
 Vcs-Git: git://anonscm.debian.org/dpkg/dpkg.git
 Standards-Version: 3.9.6
-Build-Depends: debhelper (>= 7), pkg-config, flex,
+Build-Depends: debhelper (>= 9), pkg-config, flex,
  gettext (>= 0.19), po4a (>= 0.41),
  zlib1g-dev, libbz2-dev, liblzma-dev,
  libselinux1-dev (>= 1.28-4) [linux-any],