Fix compiler warnings

Fixes the following compiler warning

```
miner.cpp: In constructor ‘COrphan::COrphan(const CTransaction*)’:
miner.cpp:69:14: warning: ‘COrphan::feeRate’ will be initialized after [-Wreorder]
     CFeeRate feeRate;
              ^
miner.cpp:68:12: warning:   ‘double COrphan::dPriority’ [-Wreorder]
     double dPriority;
            ^
miner.cpp:71:5: warning:   when initialized here [-Wreorder]
     COrphan(const CTransaction* ptxIn) : ptx(ptxIn), feeRate(0), dPriority(0)
```

src/miner.cpp

@@ -65,8 +65,8 @@ class COrphan
 public:
     const CTransaction* ptx;
     set<uint256> setDependsOn;
-    double dPriority;
     CFeeRate feeRate;
+    double dPriority;
 
     COrphan(const CTransaction* ptxIn) : ptx(ptxIn), feeRate(0), dPriority(0)
     {