Add swig to build deps for lgpio on Python 3.13

Trixie ships Python 3.13 which has no pre-built lgpio wheel,
so pip builds from source and needs swig to generate the C bindings.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

setup-pi.sh

@@ -88,7 +88,7 @@ install_system_deps() {
     sudo apt update
     sudo DEBIAN_FRONTEND=noninteractive apt install -y -o Dpkg::Options::="--force-confdef" -o Dpkg::Options::="--force-confold" \
         python3-venv python3-pip python3-dev \
-        gcc g++ make \
+        gcc g++ make swig \
         libjpeg-dev zlib1g-dev \
         libgpiod-dev gpiod \
         nginx git vim