make it work on Go 64bit also, by using explicit int size

previously, this would happen on:
go version devel +037a28ab0725 Thu Aug 22 12:13:54 2013 +0900 windows/amd64

Thu 08/22/2013 14:39:14.17 s:\workspace.go.2013\src\github.com\MalcolmJSmith\Winhello>Winhello.exe
panic: Cannot find window class.

goroutine 1 [running]:
main.WinMain(0x400000, 0x0, 0x4d5580, 0x0, 0x1, ...)
        s:/workspace.go.2013/src/github.com/MalcolmJSmith/Winhello/Winhello.go:132 +0x70d
main.main()
        s:/workspace.go.2013/src/github.com/MalcolmJSmith/Winhello/Winhello.go:77 +0x217