Reduce build warnings and enable Qt build

src/amount.h

@@ -29,6 +29,7 @@ public:
     explicit CFeeRate(const CAmount& _nSatoshisPerK) : nSatoshisPerK(_nSatoshisPerK) {}
     CFeeRate(const CAmount& nFeePaid, size_t nSize);
     CFeeRate(const CFeeRate& other) { nSatoshisPerK = other.nSatoshisPerK; }
+    CFeeRate& operator=(const CFeeRate& other) = default;
 
     CAmount GetFee(size_t size) const;                  // unit returned is satoshis
     CAmount GetFeePerK() const { return GetFee(1000); } // satoshis-per-1000-bytes