api_extensions: advertise cgroup2 support

src/lxc/api_extensions.h

@@ -37,6 +37,7 @@ static char *api_extensions[] = {
 #ifdef HAVE_STRUCT_BPF_CGROUP_DEV_CTX
 	"cgroup2_devices",
 #endif
+	"cgroup2",
 };
 
 static size_t nr_api_extensions = sizeof(api_extensions) / sizeof(*api_extensions);