imagemagick: fix build of png support when jpeg support is disabled

imagemagick: fix build of png support when jpeg support is disabled

Fixes:
http://autobuild.buildroot.net/results/d20/d20eecec8e7b947759185f77a6c8e610dd7393f3/
http://autobuild.buildroot.net/results/ee1/ee15efa8ae3f95244980810155ff7ba9f885a59d/
http://autobuild.buildroot.net/results/aa8/aa80f2fd4c7dd884ea8a1b55ad15a40c7bf40501/
http://autobuild.buildroot.net/results/9aa/9aaa044f78115d7f599ea09669c0d6bface5633e/

This combination is broken since 7.0.4-6.

Since commit a9e228f8ac26 (Implemented a private PNG caNv (canvas) chunk),
PNGsLong gets called unconditionally, but it is only defined if JPEG
support is enabled (which defines JNG_SUPPORTED), breaking the build:

MagickCore/.libs/libMagickCore-7.Q16HDRI.a(MagickCore_libMagickCore_7_Q16HDRI_la-png.o): In function `WriteOnePNGImage':
png.c:(.text+0x748d): undefined reference to `PNGsLong'
png.c:(.text+0x74b7): undefined reference to `PNGsLong'

Fix it by adding a patch unconditionally defining the helper function.

Patch submitted upstream.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>