- sdlpop doesn's support big endian CPUs, stop building for s390x

From https://build.opensuse.org/package/live_build_log/openSUSE:Factory:zSystems/sdlpop/standard/s390x

[ 21s] types.h:39:2: error: #error This program is not (yet) prepared for big endian CPUs, please contact the author.
[ 21s] 39 | #error This program is not (yet) prepared for big endian CPUs, please contact the author.