Add libseccomp-dev to urv container

Container building still failing
Add libseccomp-dev according to:
docker-library/official-images#16829
to see if it gets fixed

.github/urv-toolchain-spec_def.containerfile

@@ -19,6 +19,7 @@ RUN apt-get update -qq \
 && apt-get install -y curl \
 && apt-get install -y xz-utils \
 && apt-get install -y make \
+&& apt-get install -y libseccomp-dev \
 && curl -L -o riscv-11.2-small.tgz https://ohwr.org/project/wrpc-sw/wikis/uploads/9f9224d2249848ed3e854636de9c08dc/riscv-11.2-small.tgz \
 && tar xzvf riscv-11.2-small.tgz -C . --no-same-owner --no-overwrite-dir\
 && export CROSS_COMPILE="/riscv-11.2-small/bin/riscv32-elf-" \