explicitly type MaxTotalVotingPower to int64 (#2953)

types/validator_set.go

@@ -19,7 +19,7 @@ import (
 // x + (x >> 3) = x + x/8 = x * (1 + 0.125).
 // MaxTotalVotingPower is the largest int64 `x` with the property that `x + (x >> 3)` is
 // still in the bounds of int64.
-const MaxTotalVotingPower = 8198552921648689607
+const MaxTotalVotingPower = int64(8198552921648689607)
 
 // ValidatorSet represent a set of *Validator at a given height.
 // The validators can be fetched by address or index.