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 <[email protected]>

Author: LaurentiuM1234

include/zephyr/drivers/firmware/scmi/transport.h

@@ -13,7 +13,7 @@
 #define _INCLUDE_ZEPHYR_DRIVERS_FIRMWARE_SCMI_TRANSPORT_H_
 
 #include <zephyr/device.h>
-#include <zephyr/sys/mutex.h>
+#include <zephyr/kernel.h>
 
 struct scmi_message;
 struct scmi_channel;