add keyboards/mxss/rgblight.h from e661f1559:quantum/rgblight.h (#5461)

add keyboards/mxss/rgblight.h from e661f1559:quantum/rgblight.h (#5461)

This PR is cherry-pick from 90b9a1aa7d8a in PR #5020.

@MxBlu copied quantum/rgblight.c to keyboards/mxss/rgblight.c on July 8, 2018. At that time, I think it would be better to copy quantum/rgblight.h at the same time.
Because if someone makes a change to `quantum/rgblight.[ch]` that is incompatible with the past, it may cause problems with the compilation of keyboards/mxss.
(Like when I changed the prototype of the `rgblight_effect_XXX()` functions of rgblight.c in PR #5020.)