gd: security bump to version 2.2.4

Fixes:
CVE-2016-9317 - gdImageCreate() doesn't check for oversized images and
as such is prone to DoS vulnerabilities.
CVE-2016-6912 - double-free in gdImageWebPtr()
(without CVE):
Potential unsigned underflow in gd_interpolation.c
DOS vulnerability in gdImageCreateFromGd2Ctx()
Signed Integer Overflow gd_io.c

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/gd/gd.hash

@@ -1,2 +1,2 @@
 # Locally calculated
-sha256	746b6cbd6769a22ff3ba6f5756f3512a769bd4cdf4695dff17f4867f25fa7d3c	libgd-2.2.3.tar.xz
+sha256	137f13a7eb93ce72e32ccd7cebdab6874f8cf7ddf31d3a455a68e016ecd9e4e6	libgd-2.2.4.tar.xz

package/gd/gd.mk

@@ -4,7 +4,7 @@
 #
 ################################################################################
 
-GD_VERSION = 2.2.3
+GD_VERSION = 2.2.4
 GD_SOURCE = libgd-$(GD_VERSION).tar.xz
 GD_SITE = https://github.com/libgd/libgd/releases/download/gd-$(GD_VERSION)
 GD_INSTALL_STAGING = YES