LinuxKPI: add linux/leds.h

This is just a dummy enum and struct in order to make drivers compile
more happily as some parts are simply not hidden behind #ifdefs and
this avoids a longer-term maintenance problem.

Reviewed by: emaste
Differential Revision: https://reviews.freebsd.org/D37215

(cherry picked from commit a62d5ff5b04cb8657028c090764c3044178c4ccd)