Marathon: add missing dependency

caught by bookworm CI https://github.com/Botspot/pi-apps/actions/runs/5674577904/job/15378418240#step:3:2880 no longer pulled in by libsdl2-dev in bookworm

progress for #2401

apps/Marathon/install

@@ -5,7 +5,7 @@ version=20230529
 #made according to the instructions from the developer, but was not originally created for the Pi
 #https://github.com/Aleph-One-Marathon/alephone/wiki/Linux%20Install%20Instructions
 
-install_packages libboost-all-dev libsdl2-dev \
+install_packages libboost-all-dev libsdl2-dev libglu1-mesa-dev \
   libsdl2-image-dev libsdl2-net-dev libsdl2-ttf-dev \
   libspeexdsp-dev libzzip-dev libavcodec-dev libavformat-dev \
   libavutil-dev libswscale-dev libswresample-dev libpng-dev libcurl4-openssl-dev libminiupnpc-dev build-essential unzip || exit 1