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>