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cycleclock: Fix type conversion to match function return type (#1794)

fixes build with clang19

src/cycleclock.h:208:52: error: implicit conversion changes signedness: 'uint64_t' (aka 'unsigned long long') to 'int64_t' (aka 'long long') [-Werror,-Wsign-conversion]
  208 |   return (static_cast<uint64_t>(cycles_hi1) << 32) | cycles_lo;
      |   ~~~~~~ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~
1 error generated.