add missing header end comments

- ensures a consistent usage in header files
- also add a blank line after the copyright header where missing
- also remove orphan new-lines at the end of some files

src/uint256.h

@@ -326,4 +326,4 @@ public:
     uint64_t GetHash(const uint256& salt) const;
 };
 
-#endif
+#endif // BITCOIN_UINT256_H