Fix a non-type template parameter type mismatch

This issues a warning in gcc7.

include/rapidjson/rapidjson.h

@@ -413,7 +413,7 @@ RAPIDJSON_NAMESPACE_END
 RAPIDJSON_NAMESPACE_BEGIN
 template <bool x> struct STATIC_ASSERTION_FAILURE;
 template <> struct STATIC_ASSERTION_FAILURE<true> { enum { value = 1 }; };
-template<int x> struct StaticAssertTest {};
+template <size_t x> struct StaticAssertTest {};
 RAPIDJSON_NAMESPACE_END
 
 #define RAPIDJSON_JOIN(X, Y) RAPIDJSON_DO_JOIN(X, Y)