linker: move all linker headers to include/linker

Signed-off-by: Anas Nashif <anas.nashif@intel.com>
diff --git a/include/kernel.h b/include/kernel.h
index 667bbd6..2dff976 100644
--- a/include/kernel.h
+++ b/include/kernel.h
@@ -18,7 +18,7 @@
 #include <zephyr/types.h>
 #include <limits.h>
 #include <toolchain.h>
-#include <sections.h>
+#include <linker/sections.h>
 #include <atomic.h>
 #include <errno.h>
 #include <misc/__assert.h>