It's cumbersome for compositors to guard every
wlr_color_transform_ref() or wlr_color_transform_unref() call
behind a #if WLR_HAS_COLOR_MANAGEMENT. Moreover, none of the LCMS2
types are used in our public API.
Instead, always install the color.h header, and add a stub for
wlr_color_transform_init_linear_to_icc().
