wayland-scanner should not be native: true

[bugaevc]

Need to double-check this, but I must have misunderstood what the native: true argument does. I thought it causes find_program() to look for a program whose host is same as the build machine, but looks like it rather looks for a version whose target is the same as the build machine (and the host is too, which is implied). IOW you need this when looking up a tool to generate code for the build machine and not the host machine.