chain: define enum used as bit field as uint32_t

Bitwise logic combined with `<` with undefined signedness will
potentially results in undefined behavior. Fix this by defining the type
as a c++11 typed enum.

Fixes zcash#6017.

src/chain.h

@@ -56,7 +56,7 @@ struct CDiskBlockPos
 
 };
 
-enum BlockStatus {
+enum BlockStatus: uint32_t {
     //! Unused.
     BLOCK_VALID_UNKNOWN      =    0,