cycleclock: Fix type conversion to match function return type on riscv64

[kraj]

Fixes builds with clang

src/cycleclock.h:213:10: error: implicit conversion changes signedness: 'uint64_t' (aka 'unsigned long') to 'int64_t' (aka 'long') [-Werror,-Wsign-conversion] 213 | return cycles; | ~~ ^~ 1 error generated.

[dmah42]

thanks!