ci: builder: Install required Swig package for U-Boot prereq

New U-Boot dependency require the additional Swig package to correctly
build for some Target.

Add it to the list of Host package in the builder Dockerfile.

Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>

.gitlab/docker/builder/Dockerfile

@@ -31,6 +31,7 @@ RUN \
 	signify-openbsd \
 	subversion \
 	sudo \
+	swig \
 	unzip \
 	wget && \
     apt-get -y autoremove && \