Get rid of type warning on MSVC.

We should use json_int_t instead of int, since this can be different
depending on platform. Also MSVC warns that this can cause "loss of
information" since it's converting long long to int.

test/suites/api/test_number.c

@@ -12,7 +12,7 @@
 static void run_tests()
 {
     json_t *integer, *real;
-    int i;
+    json_int_t i;
     double d;
 
     integer = json_integer(5);