CI: fix linux build

Author: paroj

.github/workflows/ci-build.yml

@@ -16,7 +16,7 @@ jobs:
           libgles2-mesa-dev mesa-common-dev libgl1-mesa-dev libglu1-mesa-dev \
           libzzip-dev libfreetype6-dev libpugixml-dev libassimp-dev qtbase5-dev nvidia-cg-dev \
           swig3.0 mono-mcs python3-dev \
-          python3-lxml texlive-binaries texlive-base graphviz xvfb doxygen
+          python3-lxml texlive-binaries texlive-base graphviz xvfb doxygen g++-7
     - uses: actions/checkout@v2
       with:
         fetch-depth: 2