Add pngnq

Adds the pngnq utility, which is useful for quantizing RGBA images without throwing away the Alpha channel. (pngnq was at the time one of the only tools that handled quantizing Alpha correctly for PNG8, and to this day, still is one of the only tools that does it correctly.)

The weird sed line in the PKGBUILD comes directly from the AUR PKGBUILD, which does the same thing. It fixes a compile time error that stems from weird include order, apparently. Should this be moved to its own patch?