ci(image): add g++ — PyICU builds a C++ extension

PyICU's wheel build needs a C++ compiler; the image carried only gcc, so the
previous commit's PyICU install failed with "command 'g++' failed: No such
file or directory".

Author: eduralph

.github/docker/gramps-ci/Dockerfile

@@ -64,6 +64,7 @@ RUN apt-get update && apt-get install -y --no-install-recommends \
         gir1.2-atk-1.0 \
         gir1.2-gexiv2-0.10 \
         gcc \
+        g++ \
         pkg-config \
         python3-dev \
         libcairo2-dev \