[trivial] Add missing const qualifiers.

Add some const qualifiers to references that are not modified and should
be marked as const.

src/main.cpp

@@ -1603,7 +1603,7 @@ bool GetTransaction(const uint256 &hash, CTransaction &txOut, const Consensus::P
     if (fAllowSlow) { // use coin database to locate block that contains transaction, and scan it
         int nHeight = -1;
         {
-            CCoinsViewCache &view = *pcoinsTip;
+            const CCoinsViewCache& view = *pcoinsTip;
             const CCoins* coins = view.AccessCoins(hash);
             if (coins)
                 nHeight = coins->nHeight;