debian: add less as a build dependency

For builds that run on hermetic environments, it's possible that the "less"
package is not installed by default, yet it's needed for tests to pass after
revision bf5e13e38390 (which sets less as the fallback pager).

contrib/debian/control

@@ -5,6 +5,7 b' Maintainer: Mercurial Developers <mercur'
 Build-Depends:
  debhelper (>= 9),
  dh-python,
+ less,
  netbase,
  python-all,
  python-all-dev,