kernel: migrate includes to <zephyr/...>

In order to bring consistency in-tree, migrate all kernel code to the
new prefix <zephyr/...>. Note that the conversion has been scripted,
refer to zephyrproject-rtos#45388 for more details.

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>
diff --git a/kernel/include/kernel_internal.h b/kernel/include/kernel_internal.h
index e6ba285..af9cd54 100644
--- a/kernel/include/kernel_internal.h
+++ b/kernel/include/kernel_internal.h
@@ -14,7 +14,7 @@
 #ifndef ZEPHYR_KERNEL_INCLUDE_KERNEL_INTERNAL_H_
 #define ZEPHYR_KERNEL_INCLUDE_KERNEL_INTERNAL_H_
 
-#include <kernel.h>
+#include <zephyr/kernel.h>
 #include <kernel_arch_interface.h>
 #include <string.h>