Make XRPAmount constructor explicit:

Remove the implicit conversion from int64 to XRPAmount. The motivation for this
was noticing that many calls to `to_string` with an integer parameter type were
calling the wrong `to_string` function. Since the calls were not prefixed with
`std::`, and there is no ADL to call `std::to_string`, this was converting the
int to an `XRPAmount` and calling `to_string(XRPAmount)`.

Since `to_string(XRPAmount)` did the same thing as `to_string(int)` this error
went undetected.

src/ripple/protocol/STAmount.h

@@ -105,6 +105,7 @@ public:
     STAmount (SField const& name, Issue const& issue,
         std::uint64_t mantissa = 0, int exponent = 0, bool negative = false);
 
+    explicit
     STAmount (std::uint64_t mantissa = 0, bool negative = false);
 
     STAmount (Issue const& issue, std::uint64_t mantissa = 0, int exponent = 0,