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

(cherry picked from commit 689410f90884759b1d10231fc271f1a416030577)