x11-wm/river: restore /usr fix accidentally removed in 1d4d1fedb94d

The manpage still has /usr refernces e.g.,

/usr/include/xkbcommon/xkbcommon-keysyms.h
/usr/include/linux/input-event-codes.h

(cherry picked from commit 0bef5c19453e3ed661f845a20fd1c4979112642c)