contrib: png: update to 1.6.37

This release fixed CVE-2019-7317, although our png module is not using this API.

contrib/src/png/SHA512SUMS

-a86ee977df69748e5039fb0ead883f1d3f88b8a701fa24cf8e62dd77c5871bb46397d794fa33ec1d0be1ac488246832ad79d0e6117ac093bdce1b2a1cfcb2bb0  libpng-1.6.36.tar.xz
+59e8c1059013497ae616a14c3abbe239322d3873c6ded0912403fc62fb260561768230b6ab997e2cccc3b868c09f539fd13635616b9fa0dd6279a3f63ec7e074  libpng-1.6.37.tar.xz

contrib/src/png/rules.mak

 # PNG
-PNG_VERSION := 1.6.36
+PNG_VERSION := 1.6.37
 PNG_URL := $(SF)/libpng/libpng16/$(PNG_VERSION)/libpng-$(PNG_VERSION).tar.xz
 
 PKGS += png