kunit: fix -Wunused-function warning for __kunit_fail_current_test

When CONFIG_KUNIT is not enabled, __kunit_fail_current_test() an empty
static function.

But GCC complains about unused static functions, *unless* they're static
inline.  So add inline to make GCC happy.

Fixes: 359a3760

 ("kunit: support failure from dynamic analysis tools")
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Daniel Latypov <dlatypov@google.com>
Reviewed-by: Brendan Higgins <brendanhiggins@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>