Update SUMMARY.md

SUMMARY.md

@@ -48,6 +48,7 @@
 * [Concepts](Concepts/README.md)
     * [Per-CPU variables](Concepts/per-cpu.md)
     * [Cpumasks](Concepts/cpumask.md)
+    * [The initcall mechanism](Concepts/initcall.md)
 * [Data Structures in the Linux Kernel](DataStructures/README.md)
     * [Doubly linked list](DataStructures/dlist.md)
     * [Radix tree](DataStructures/radix-tree.md)