kernel: Fix compile warning with _impl_k_object_alloc

We get the following warning with CONFIG_DYNAMIC_OBJECTS=n in
_impl_k_object_alloc:

include/kernel.h:322:57: warning: unused parameter ‘otype’ [-Wunused-parameter]
 static inline void *_impl_k_object_alloc(enum k_objects otype)
                                                         ^~~~~
Simple fix is to ARG_UNUSED otype.

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>