ci: Add gdb to some CI images

This will be useful in general, for manually debugging failures if we
need (without having to rebuild the CI images to do so).

It will also be useful for the `assert-msg-test` test, which is being
reworked in !2767.

Signed-off-by: Philip Withnall <pwithnall@endlessos.org>

Helps: #1434

.gitlab-ci/debian-stable.Dockerfile

@@ -13,6 +13,7 @@ RUN apt-get update -qq && apt-get install --no-install-recommends -qq -y \
     flake8 \
     fuse \
     gcc \
+    gdb \
     g++ \
     gettext \
     git \

.gitlab-ci/fedora.Dockerfile

@@ -13,6 +13,7 @@ RUN dnf -y update \
     fuse \
     gcc \
     gcc-c++ \
+    gdb \
     gettext \
     git \
     glibc-devel \