libpng: update to version 1.6.37

Fixes CVE-2019-7317

Signed-off-by: Jan Pavlinec <jan.pavlinec@nic.cz>

libs/libpng/Makefile

@@ -8,12 +8,12 @@
 include $(TOPDIR)/rules.mk
 
 PKG_NAME:=libpng
-PKG_VERSION:=1.6.36
+PKG_VERSION:=1.6.37
 PKG_RELEASE:=1
 
 PKG_SOURCE:=$(PKG_NAME)-$(PKG_VERSION).tar.xz
 PKG_SOURCE_URL:=@SF/libpng
-PKG_HASH:=eceb924c1fa6b79172fdfd008d335f0e59172a86a66481e09d4089df872aa319
+PKG_HASH:=505e70834d35383537b6491e7ae8641f1a4bed1876dbfe361201fc80868d88ca
 
 PKG_MAINTAINER:=Jo-Philipp Wich <jo@mein.io>
 PKG_LICENSE:=Libpng GPL-2.0+ BSD-3-Clause