Install libffi-dev to fix xKAN-meta_testing Docker image build

Dockerfile.metadata

@@ -3,7 +3,7 @@ FROM mono:latest
 RUN /bin/sed -i 's/^mozilla\/DST_Root_CA_X3.crt$/!mozilla\/DST_Root_CA_X3.crt/' /etc/ca-certificates.conf && \
     /usr/sbin/update-ca-certificates
 RUN apt-get update && \
-    apt-get install -y --no-install-recommends python3 python3-pip python3-setuptools git build-essential python3-dev && \
+    apt-get install -y --no-install-recommends python3 python3-pip python3-setuptools git build-essential python3-dev libffi-dev && \
     apt-get clean
 RUN pip3 install 'git+https://github.com/KSP-CKAN/NetKAN-Infra#subdirectory=netkan'
 RUN pip3 install 'git+https://github.com/KSP-CKAN/xKAN-meta_testing'