emulators/{ppsspp,libretro-ppsspp,ppsspp-qt5}: fix build on arm64
Arm64 requires bundled libpng17 module. Removing it from EXCLUDE
unbreaks build on arm64 platform.
PR: 287572
Reported by: Kevin Reinholz <kreinholz@gmail.com> (maintainer)
MFH: 2025Q3