os: fdtable: include zephyr/kernel.h for struct k_mutex

Was getting the following error:

include/zephyr/sys/fdtable.h:150:38: warning: 'struct k_mutex'
declared inside parameter list will not be visible outside of
this definition or declaration
  150 |                               struct k_mutex **lock);
      |                                      ^~~~~~~

(#51667) tried to fix this by including `zephyr/sys/mutex.h`,
but `struct k_mutex` is defined in `zephyr/kernel.h`, so
include the latter instead.

Signed-off-by: Yong Cong Sin <ycsin@meta.com>

include/zephyr/sys/fdtable.h

@@ -11,7 +11,7 @@
 
 /* FIXME: For native_posix ssize_t, off_t. */
 #include <zephyr/fs/fs.h>
-#include <zephyr/sys/mutex.h>
+#include <zephyr/kernel.h>
 #include <zephyr/sys/util.h>
 
 /* File mode bits */