fix imcompatible pointer type casting

`objc_object` is `id` type.
But, NUM2ULONG returns `unsigned long` type.

This incompatible cast causes following warning:

```log
rb_cairo_surface.c:1281:31: warning: incompatible integer to pointer conversion
assigning to 'id' (aka 'struct objc_object *') from 'unsigned long'
[-Wint-conversion]
objc_object = NUM2ULONG (rb_objc_pointer);
            ^
            ~~~~~~~~~~~~~~~~~~~~~~~~~~~
1 warning generated.
```