package/libpng: cleanup NEON support options

Suggested by Thomas:
http://lists.busybox.net/pipermail/buildroot/2017-April/189086.html



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