arm64-v8a build fail with missing neon intrinsics

[netx123]

Hi, your distribution is missing the libpng arm directory, so when building for arm64-v8a architectures which support NEON, the build fails with 'undefined reference to 'png_init_filter_functions_neon'

[sreejith-mohanan]

Changing the line number 130 in pngpriv.h form

define PNG_ARM_NEON_OPT 2

to

define PNG_ARM_NEON_OPT 0

will avoid neon optimization in arm and will fix the bug

[mlknz]

Fix worked for me.

Edit:
@julienr could you repair distribution please? Adding arm directory to sources and Android.mk solves issue.

Edit 2:
I have created PR which fixes the issue for me.
#11

[julienr]

@mlknz Merged the PR, thanks !