libpng: bump to version 1.6.31

Signed-off-by: Adam Duskett <Adamduskett@outlook.com>
[Thomas: fix hash file comment, noticed by Baruch.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

package/libpng/libpng.hash

@@ -1,4 +1,4 @@
-# From http://sourceforge.net/projects/libpng/files/libpng16/1.6.29/
-sha1 7dbe6a5088b938545fc0857c507d4e0cf5d9023e  libpng-1.6.29.tar.xz
+# From http://sourceforge.net/projects/libpng/files/libpng16/1.6.31/
+sha1 de695064363df331734466981ef7f6546ef516bf  libpng-1.6.31.tar.xz
 # Locally computed:
-sha256 4245b684e8fe829ebb76186327bb37ce5a639938b219882b53d64bd3cfc5f239  libpng-1.6.29.tar.xz
+sha256 232a602de04916b2b5ce6f901829caf419519e6a16cc9cd7c1c91187d3ee8b41  libpng-1.6.31.tar.xz

package/libpng/libpng.mk

@@ -4,7 +4,7 @@
 #
 ################################################################################
 
-LIBPNG_VERSION = 1.6.29
+LIBPNG_VERSION = 1.6.31
 LIBPNG_SERIES = 16
 LIBPNG_SOURCE = libpng-$(LIBPNG_VERSION).tar.xz
 LIBPNG_SITE = http://downloads.sourceforge.net/project/libpng/libpng${LIBPNG_SERIES}/$(LIBPNG_VERSION)