fix(build): uint is not defined on Windows platform (#4731)

Fix the Windows build by using `unsigned int` (instead of `uint`).

The error, introduced by #4618, looks something like:
  rpc\impl\RPCHelpers.h(299,5): error C2061: syntax error: identifier
  'uint' (compiling source file app\ledger\Ledger.cpp)

src/ripple/rpc/impl/RPCHelpers.h

@@ -296,7 +296,7 @@ std::pair<PublicKey, SecretKey>
 keypairForSignature(
     Json::Value const& params,
     Json::Value& error,
-    uint apiVersion = apiVersionIfUnspecified);
+    unsigned int apiVersion = apiVersionIfUnspecified);
 /** Helper to parse submit_mode parameter to RPC submit.
  *
  * @param params RPC parameters