[BUG] apngopt.cpp miss a header

[Jristz]

apngopt.cpp call memcpy and other, this one need #cstring" or "string.h" to
work, else fail to build in ANY system.

so the latest 1.3 is unbuildable.

--
Pablo Lezaeta

[Max Stepin]

Thanks. I'll put it back.

I had plans to release 1.4 tomorrow, so just wait a little bit.