x11/mako: unbreak build on DragonFly

../notification.c: In function 'get_binding':
../notification.c:275:14: error: 'BTN_LEFT' undeclared (first use in this function)

275 |         case BTN_LEFT:
    |              ^~~~~~~~

../notification.c:275:14: note: each undeclared identifier is reported only once for each function it appears in
../notification.c:277:14: error: 'BTN_RIGHT' undeclared (first use in this function)

277 |         case BTN_RIGHT:
    |              ^~~~~~~~~

../notification.c:279:14: error: 'BTN_MIDDLE' undeclared (first use in this function)

279 |         case BTN_MIDDLE:
    |              ^~~~~~~~~~

Reported by: DPorts