Update img3unpack/img3.c

"img3.h" casts its variables as unsigned int, yet in "img3.c", it asks for long signed int. The solution was to either change "img3.h" to cast the variables as long signed int, or change "img3.c" to ask for an unsigned int. I chose the later.
Merge pull request #15 from renegadezombies/patch-1

Update img3unpack/img3.c