Fix an int->pointer cast warning in windows.c.

If I increase clang-cl's warning pickiness, it starts objecting to my
cast to HMENU from a (potentially, in 64 bits) smaller integer type.

Actually I don't think there's a problem there - all the integer ids I
cast to HMENU are nice small numbers and a widening cast is just fine.
But I can suppress the warning by using INT_PTR instead of int in the
prototype for mkctrl, so it's easiest just to do that.