Update crc32c_round_up.h - gcc -Werror=sign-conversion supression

[inobelar]

Greetings! I spotted the next warning (using gcc (GCC) 15.1.1 20250425) (in my case with -Werror & other flags for strictness - its an error):

crc32c-main/src/crc32c_round_up.h:19:30: error: unsigned conversion from ‘int’ to ‘uintptr_t’ {aka ‘long unsigned int’} changes value from ‘-4’ to ‘18446744073709551612’ [-Werror=sign-conversion]
   19 |   return (pointer + (N - 1)) & ~(N - 1);
      |          ~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~

This warning is caused by implicit signed-to-unsigned conversion when ~(N - 1) is evaluated. The result of the ~ operator on an int may be negative (e.g., ~3 = -4), and converting that to uintptr_t results in a large positive number due to two's complement representation.

That's why I added explicit cast of (N - 1) into uintptr_t before applying the bitwise-not.

Why this fix works and dont break everything:

• N is of type int, so N - 1 is also an int.
• ~(N - 1) thus results in an int, which might be negative.
• When used in the bitwise & with pointer (which is uintptr_t), the int gets implicitly converted to uintptr_t. If it’s negative, the result is a large positive value (e.g., -4 becomes 0xFFFFFFFFFFFFFFFC on a 64-bit system).
• Casting (N - 1) to uintptr_t before applying ~ ensures the operation is done in unsigned arithmetic, resulting in the correct bitmask without sign-conversion issues.

With this small fix everything was build successfully without any other warnings/errors (on x86_64).