selinux: fix compile errors

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

introduced by 3f48345f

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]

(cherry picked from commit 4d4c90df)

src/security/security_selinux.c

@@ -2506,13 +2506,12 @@ virSecuritySELinuxGetSecurityMountOptions(virSecurityManagerPtr mgr,
     return opts;
 }
 
-static char *
-virSecuritySELinuxDomainSetDirLabel(virSecurityManagerPtr mgr,
+static int
+virSecuritySELinuxDomainSetDirLabel(virSecurityManagerPtr mgr ATTRIBUTE_UNUSED,
                                     virDomainDefPtr def,
                                     const char *path)
 {
     virSecurityLabelDefPtr seclabel;
-    int ret = -1;
 
     seclabel = virDomainDefGetSecurityLabelDef(def, SECURITY_SELINUX_NAME);
     if (!seclabel || !seclabel->relabel)