Eliminate signed/unsigned comparison warning

src/main.cpp

@@ -2849,7 +2849,7 @@ bool AcceptBlock(CBlock& block, CValidationState& state, CBlockIndex** ppindex,
     // blocks which are too close in height to the tip.  Apply this test
     // regardless of whether pruning is enabled; it should generally be safe to
     // not process unrequested blocks.
-    bool fTooFarAhead = (pindex->nHeight - chainActive.Height()) > MIN_BLOCKS_TO_KEEP;
+    bool fTooFarAhead = (pindex->nHeight > int(chainActive.Height() + MIN_BLOCKS_TO_KEEP));
 
     // TODO: deal better with return value and error conditions for duplicate
     // and unrequested blocks.