apps/passwd.c: 32 bits are sufficient to hold ROUNDS_MAX.

Even though C standard defines 'z' modifier, recent mingw compilers break
the contract by defining __STDC_VERSION__ with non-compliant MSVCRT.DLL.
In other words we can't use %zu with mingw, but insteadl of cooking

Reviewed-by: Tim Hudson <tjh@openssl.org>