Update difficulty.cpp

Changed size_t  to match solvetime

src/cryptonote_basic/difficulty.cpp

@@ -219,7 +219,7 @@ namespace cryptonote {
     uint64_t difficulty(0), next_difficulty(0);
 
     // Loop through N most recent blocks. N is most recently solved block.
-    for (size_t i = 1; i <= N; i++) {
+    for (int64_t i = 1; i <= N; i++) {
       solveTime = static_cast<int64_t>(timestamps[i]) - static_cast<int64_t>(timestamps[i - 1]);
       solveTime = std::min<int64_t>((T * 7), std::max<int64_t>(solveTime, (-7 * T)));
       difficulty = cumulative_difficulties[i] - cumulative_difficulties[i - 1];