Add C++ compiler into GitLab CI Ubuntu image (!60)

Next the Ubuntu image CI job is failing without a C++ compiler like
this:

    checking whether to enable maintainer-specific portions of Makefiles... yes
    checking for g++... no
    checking for c++... no
    checking for gpp... no
    checking for aCC... no
    checking for CC... no
    checking for cxx... no
    checking for cc++... no
    checking for cl.exe... no
    checking for FCC... no
    checking for KCC... no
    checking for RCC... no
    checking for xlC_r... no
    checking for xlC... no
    checking whether the C++ compiler works... no
    configure: error: in `/builds/mfleetwo/gparted':
    configure: error: C++ compiler cannot create executables
    See `config.log' for more details
    ...
    ERROR: Job failed: exit code 1

The published "Ubuntu" docker image has been updated to Ubuntu 20.04 LTS
and must no longer include the build tools by default, or not be a
dependency of any of the other installed packages.  Explicitly install
build-essential to get the C++ compiler [1].  Also don't list make as
build-essential includes it.

[1] Installing the GNU C compiler and GNU C++ compiler
    https://help.ubuntu.com/community/InstallingCompilers

Closes !60 - Fix GitLab CI job failures following Ubuntu docker image
             updates

.gitlab-ci.yml

@@ -25,9 +25,9 @@ stages:
   before_script:
     - export DEBIAN_FRONTEND=noninteractive
     - apt-get update
-    - apt-get install -y gnome-common yelp-tools libglib2.0-dev-bin
-                         uuid-dev libparted-dev libgtkmm-3.0-dev make
-                         policykit-1
+    - apt-get install -y build-essential gnome-common yelp-tools
+                         libglib2.0-dev-bin uuid-dev libparted-dev
+                         libgtkmm-3.0-dev policykit-1
     # Extra packages only needed during the test stage.
     - apt-get install -y btrfs-progs e2fsprogs f2fs-tools dosfstools mtools
                          hfsutils hfsprogs jfsutils util-linux cryptsetup-bin