firmware: scmi: transport.h: switch to including kernel.h

Toggling `CONFIG_USERSPACE` to `y` results in compilation
errors caused by the fact that various k_* functions don't
have their prototypes included in the SCMI sources (e.g:
`k_mutex_*`, `k_sem_*`, `k_is_pre_kernel()`).

To fix this, switch to using the `kernel.h` header instead
of `sys/mutex.h`. Previously, this worked because `sys/mutex.h`
included `kernel.h` if `CONFIG_USERSPACE` was set to `n`.

Signed-off-by: Laurentiu Mihalcea <laurentiu.mihalcea@nxp.com>