Update libpng to v1.6.34 for cmake

The libpng has been updated from v1.2.53 to v1.6.34 in PR 18299.
However, the cmake version of libpng has not been updated yet.
This fix updates the libpng for cmake to v1.6.34.

The fix is tested with cmake on linux:
```
tensorflow/tools/ci_build/ci_build.sh CMAKE tensorflow/tools/ci_build/builds/cmake.sh
```

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>