package/pngquant: bump version to 2.15.1

Changelog:
https://github.com/kornelski/pngquant/blob/master/CHANGELOG

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

package/pngquant/pngquant.hash

@@ -1,4 +1,4 @@
 # From https://pngquant.org/releases.html
-sha256  4b911a11aa0c35d364b608c917d13002126185c8c314ba4aa706b62fd6a95a7a  pngquant-2.13.1-src.tar.gz
+sha256  718aabbc346b82ed93564d8e757b346988d86de268ee03d2904207cd5d64c829  pngquant-2.15.1-src.tar.gz
 # Locally computed
 sha256  e4f467f7abf860b4e620f5f60fdd88bc3a63a0fdb98a481fb002b5e511c9b826  COPYRIGHT

package/pngquant/pngquant.mk

@@ -4,7 +4,7 @@
 #
 ################################################################################
 
-PNGQUANT_VERSION = 2.13.1
+PNGQUANT_VERSION = 2.15.1
 PNGQUANT_SOURCE = pngquant-$(PNGQUANT_VERSION)-src.tar.gz
 PNGQUANT_SITE = https://pngquant.org
 PNGQUANT_LICENSE = GPL-3.0+