fix missing <string.h> header in io_png.c
fix missing <string.h> header in io_png.c

mnhrdt/s2p,jmichel-otb/s2p,dyoussef/s2p,jmichel-otb/s2p,MISS3D/s2p,MISS3D/s2p,carlodef/s2p,dyoussef/s2p,carlodef/s2p,MISS3D/s2p,jmichel-otb/s2p,dyoussef/s2p,MISS3D/s2p,mnhrdt/s2p,jmichel-otb/s2p,dyoussef/s2p