package/lcdproc: add libpng optional dependency

Fixes:
http://autobuild.buildroot.net/results/b68/b68f54404a7d412d639ebb6d052412eea6ee3bf1



Signed-off-by: Romain Naour <romain.naour@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>