docs: add 'swig' to the Ubuntu 22.04 prerequisites

When building for RockPi4 using Ubuntu 22.04 we need to have the host
package 'swig' installed as well.

Signed-off-by: Joakim Bech <joakim.bech@linaro.org>
Acked-by: Jens Wiklander <jens.wiklander@linaro.org>
Acked-by: Jerome Forissier <jerome.forissier@linaro.org>

building/prerequisites.rst

@@ -56,6 +56,7 @@ available.
               python3-serial \
               python-is-python3 \
               rsync \
+              swig \
               unzip \
               uuid-dev \
               xdg-utils \