tests/docker: Add flex/bison to `debian-hexagon-cross`

tests/docker: Add flex/bison to `debian-hexagon-cross`

debian-hexagon-cross contains two images, one to build the toolchain
used for building the Hexagon tests themselves, and one image to build
QEMU and run the tests.

This commit adds flex/bison to the final image that builds QEMU so that
it can also build idef-parser.

Note: This container is not built by the CI and needs to be rebuilt and
updated manually.

Signed-off-by: Anton Johansson <anjo@rev.ng>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20221014223642.147845-1-anjo@rev.ng>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20221027183637.2772968-8-alex.bennee@linaro.org>