[tlbuild] libpng on aarch64

[Mojca Miklavec]

Hi,

the latest libpng update is breaking the linux-aarch64 build:

../../../libs/libpng/libpng-src/arm/arm_init.c:39:6: error: #error
"PNG_ARM_NEON_CHECK_SUPPORTED must not be defined on this platform"
 #    error "PNG_ARM_NEON_CHECK_SUPPORTED must not be defined on this platform"
      ^~~~~
../../../libs/libpng/libpng-src/arm/arm_init.c:47:12: warning:
'png_have_neon' used but never defined
 static int png_have_neon(png_structp png_ptr);
            ^~~~~~~~~~~~~

(I can try to look into it a bit later.)

Mojca