fix anonymous argument error in backup_sleep_without_gvl for some compilers

[sitano]

Our CI/CD pipeline produces in its images:

database.c: In function 'backup_sleep_without_gvl':
database.c:425:1: error: parameter name omitted
 void *backup_sleep_without_gvl(void *) {
 ^~~~

as an alternative it can be replaced with accomp attr:

void *backup_sleep_without_gvl(__attribute__((unused)) void *unused)