package/libpng: security bump to version 1.6.37

Fixes the following security issue:

CVE-2019-7317: png_image_free in png.c in libpng 1.6.36 has a use-after-free
because png_image_free_function is called under png_safe_execute.

Update license hash for a change in copyright year and typo fixes.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>