kunit: Export kunit_running()

Using kunit_fail_current_test() in a loadable module causes a link
error like:

ERROR: modpost: "kunit_running" [drivers/gpu/drm/vc4/vc4.ko] undefined!

Export the symbol to allow using it from modules.

Fixes: da43ff04 ("drm/vc4: tests: Fail the current test if we access a register")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: David Gow <davidgow@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>