Fix `-Wint-conversion` error on 32-bit arches.

```
../src/gxr-context.c: In function '_destroy_session':
../src/gxr-context.c:626:17: error: assignment to 'XrSession' {aka 'long
long unsigned int'} from 'void *' makes integer from pointer without a
cast [-Wint-conversion]
  626 |   self->session = NULL;
      |                 ^
```