include: add missing kernel.h include

Some files make use of Kernel APIs without including kernel.h, fix this
problem.

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>

subsys/fs/fs.c

diff --git a/subsys/fs/fs.c b/subsys/fs/fs.c
index 91aedc1..edd3922 100644
--- a/subsys/fs/fs.c
+++ b/subsys/fs/fs.c
@@ -11,6 +11,7 @@
 #include <zephyr/types.h>
 #include <errno.h>
 #include <zephyr/init.h>
+#include <zephyr/kernel.h>
 #include <zephyr/fs/fs.h>
 #include <zephyr/fs/fs_sys.h>
 #include <zephyr/sys/check.h>