[trivial] Add missing const qualifiers.

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

src/txmempool.h

@@ -672,10 +672,10 @@ private:
 class CCoinsViewMemPool : public CCoinsViewBacked
 {
 protected:
-    CTxMemPool &mempool;
+    const CTxMemPool& mempool;
 
 public:
-    CCoinsViewMemPool(CCoinsView *baseIn, CTxMemPool &mempoolIn);
+    CCoinsViewMemPool(CCoinsView* baseIn, const CTxMemPool& mempoolIn);
     bool GetCoins(const uint256 &txid, CCoins &coins) const;
     bool HaveCoins(const uint256 &txid) const;
 };