Fix JSON_INTEGER overflow on Windows by changing datatype to 'long long'

Signed-off-by: Jens Axboe <axboe@fb.com>
diff --git a/json.h b/json.h
index 2a798ce..081afd6 100644
--- a/json.h
+++ b/json.h
@@ -14,7 +14,7 @@
 struct json_value {
 	int type;
 	union {
-		long integer_number;
+		long long integer_number;
 		double float_number;
 		char *string;
 		struct json_object *object;