Tracing: Tracing/Kernel dependency issue fix

Added forward declarations of types declared
throughout kernel.h at the start of
kernel.h. With this change it is possible to
include tracing/tracing.h early in kernel.h
and use the tracing functionality in
kernel.h functions without compile errors
which would result if tracing/tracing.h
was included at the end of kernel.h
(type dependencies).

Signed-off-by: Torbjörn Leksell <torbjorn.leksell@percepio.com>