linuxkpi: Define DEFINE_WD_CLASS(name) in <linux/ww_mutex.h>

It is defined as a synonymous to DEFINE_WW_CLASS(name).

Reviewed by: manu
Approved by: manu
Differential Revision: https://reviews.freebsd.org/D38151

(cherry picked from commit d9f1cb6774f1785544c2fbeba476d4842967e804)