Add an explicit cast to double to squash a -Wimplicit-int-float-conversion warning.

Though we will no longer be comparing exactly against INT64_MAX, this is ok
because any value of that magnitude stored in a double will *also* have been
rounded up, so the comparison will work appropriately.

json_object.c

@@ -701,7 +701,9 @@ int64_t json_object_get_int64(const struct json_object *jso)
 	case json_type_int:
 		return jso->o.c_int64;
 	case json_type_double:
-		if (jso->o.c_double >= INT64_MAX)
+		// INT64_MAX can't be exactly represented as a double
+		// so cast to tell the compiler it's ok to round up.
+		if (jso->o.c_double >= (double)INT64_MAX)
 			return INT64_MAX;
 		if (jso->o.c_double <= INT64_MIN)
 			return INT64_MIN;