Make cellsize a char.

Make cellsize a char.

Apparently new versions of GCC get grumpy if the possible range for a sprintf()
call exceeds MAXINT, which would never happen in actuality here due to the size
of the puzzles we're dealing with... but the compiler doesn't know that, of
course, so thinks that something may have gone horribly awry.  Changing it to a
char solves the problem neatly.