debian: Add missing Build-Depends for restriction formula support

Although these are really not needed, as they are available already from
Debian stable, it makes it obvious for people who might want to backport
dpkg to older suites or distributions.

Warned-by; lintian

debian/changelog

@@ -47,6 +47,8 @@ dpkg (1.18.4) UNRELEASED; urgency=medium
       to generate, default for now is development documentation.
     - Try to use AM_GNU_GETTEXT_REQUIRE_VERSION to benefit from the latest
       installed gettext version, while guaranteeing a minimal required version.
+  * Packaging:
+    - Add missing Build-Depends for restriction formula support.
   * Documentation:
     - Move description for “target architecture” from the dpkg-architecture(1)
       ‘-A’ option to the TERMS section. Closes: #799046

debian/control

@@ -9,7 +9,8 @@ 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 (>= 9), pkg-config, flex,
+Build-Depends: dpkg-dev (>= 1.17.14), debhelper (>= 9.20141010),
+ pkg-config, flex,
  gettext (>= 0.19), po4a (>= 0.41),
  zlib1g-dev, libbz2-dev, liblzma-dev,
  libselinux1-dev (>= 1.28-4) [linux-any],