Add cast to eliminate warning
Also, while here, perform the inverse cast to uint instead of int.
Without this patch I get:
json-glib/json-scanner.c: In function 'json_scanner_get_token_ll':
json-glib/json-scanner.c:1309:30: warning: assignment to 'gpointer' {aka 'void *'} from 'guint' {aka 'unsigned int'} makes pointer from integer without a cast [-Wint-conversion]
1309 | value.v_symbol = json_symbols[i].token;
| ^