kernel: add missing include

This syncs the cs_main definition/declaration.

Noticed when experimenting with the external visibility of cs_main.

src/kernel/cs_main.cpp

@@ -2,6 +2,7 @@
 // Distributed under the MIT software license, see the accompanying
 // file COPYING or http://www.opensource.org/licenses/mit-license.php.
 
+#include <kernel/cs_main.h>
 #include <sync.h>
 
 RecursiveMutex cs_main;