Allow libpng to be compiled with `-mfpu=neon`

This is needed because currently compiling KOReader with the `-mfpu=neon` flag will make it crash with unresolved symbol errors.

This change is otherwise innocuous because of the `PNG_ARM_NEON_OPT` check in both new source files. So it will still compile fine both for non-NEON ARM and for non-ARM architectures.