selinux: fix compile errors

selinux: fix compile errors

Remove unused variable, tag unused parameter and adjust return type.

introduced by 3f48345f7ec0c44b8ce2371cab454d2118f830b4

CC     security/libvirt_security_manager_la-security_selinux.lo
security/security_selinux.c: In function 'virSecuritySELinuxDomainSetDirLabel':
security/security_selinux.c:2520:5: error: return makes pointer from integer without a cast [-Werror]
security/security_selinux.c:2514:9: error: unused variable 'ret' [-Werror=unused-variable]
security/security_selinux.c:2509:59: error: unused parameter 'mgr' [-Werror=unused-parameter]