Fix unused warning with C++17 [[maybe_unused]] decoration

Get rid of the warning: ``` In file included from /usr/src/xmrig/src/crypto/cn/CnHash.cpp:29: /usr/src/xmrig/src/crypto/cn/CryptoNight_x86.h: In function ‘void do_flex_skein_hash(const uint8_t*, size_t, uint8_t*)’: /usr/src/xmrig/src/crypto/cn/CryptoNight_x86.h:80:9: warning: unused variable ‘r’ [-Wunused-variable] 80 | int r = skein_hash(512, input, 8 * len, (uint8_t*)output); | ^ ```