Change virCgroupGetAppRoot stub on non-Linux to avoid unused param warning

Fully stub out the virCgroupGetAppRoot method as done with other
methods in the file, rather than just the body. This lets us
annotate the unused parameter to avoid a warning

src/util/cgroup.c

@@ -974,14 +974,17 @@ int virCgroupForDriver(const char *name ATTRIBUTE_UNUSED,
 *
 * Returns 0 on success
 */
+#if defined HAVE_MNTENT_H && defined HAVE_GETMNTENT_R
 int virCgroupGetAppRoot(virCgroupPtr *group)
 {
-#if defined HAVE_MNTENT_H && defined HAVE_GETMNTENT_R
     return virCgroupNew("/", group);
+}
 #else
+int virCgroupGetAppRoot(virCgroupPtr *group ATTRIBUTE_UNUSED)
+{
     return -ENXIO;
-#endif
 }
+#endif
 
 /**
  * virCgroupForDomain: