Add asciidoctor to Debian build dependencies

asciidoctor is needed to generate manual.html for the Debian package. This did
not cause build failures before because CMake was (incorrectly) installing the
manual.html checked into source control, not the one generated by the build
script.

debian/changelog

@@ -1,4 +1,4 @@
-darkradiant (2.14.0~focal1) focal; urgency=medium
+darkradiant (2.14.0~focal2) focal; urgency=medium
 
   * New major release on all platforms.
   * Multi-selection support in Entity Inspector.

debian/control

@@ -2,7 +2,7 @@ Source: darkradiant
 Section: editors
 Priority: optional
 Maintainer: Matthew Mott <orbweaver3d@gmail.com>
-Build-Depends: debhelper (>= 10), cmake (>= 3.12), pkg-config, libxml2-dev, libglew-dev, python-dev, libvorbis-dev, libopenal-dev, libalut-dev, libjpeg-dev, libftgl-dev, libwxbase3.0-dev, libwxgtk3.0-gtk3-dev, libsigc++-2.0-dev, libglib2.0-dev, libeigen3-dev, libgit2-dev
+Build-Depends: debhelper (>= 10), cmake (>= 3.12), pkg-config, asciidoctor, libxml2-dev, libglew-dev, python-dev, libvorbis-dev, libopenal-dev, libalut-dev, libjpeg-dev, libftgl-dev, libwxbase3.0-dev, libwxgtk3.0-gtk3-dev, libsigc++-2.0-dev, libglib2.0-dev, libeigen3-dev, libgit2-dev
 Standards-Version: 4.5.1
 
 Package: darkradiant