kunit: Add example of kunit_activate_static_stub() with pointer-to-function

kunit: Add example of kunit_activate_static_stub() with pointer-to-function

Adds a variant of example_static_stub_test() that shows use of a
pointer-to-function with kunit_activate_static_stub().

A const pointer to the add_one() function is declared. This
pointer-to-function is passed to kunit_activate_static_stub() and
kunit_deactivate_static_stub() instead of passing add_one directly.

Signed-off-by: Richard Fitzgerald <rf@opensource.cirrus.com>
Reviewed-by: David Gow <davidgow@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>