cmake: fix ninja build
cmake: fix ninja build

When using the ninja generator:

  $ cmake -G Ninja <dir>
  $ ninja -C <dir>

This commit fixes the warning:

ninja: warning: multiple rules generate bin/wayland-desktop-shell-protocol.c.  builds involving this target will not be correct; continuing anyway [-w dupbuild=warn]

1ace/sway,mikkeloscar/sway,ascent12/sway,christophgysin/sway,ascent12/sway,sleep-walker/sway,SirCmpwn/sway,1ace/sway,christophgysin/sway,sleep-walker/sway,crondog/sway,colemickens/sway,taiyu-len/sway,ptMuta/sway,johalun/sway,colemickens/sway,1ace/sway,crondog/sway,taiyu-len/sway,4e554c4c/sway,4e554c4c/sway,mikkeloscar/sway,ascent12/sway,colemickens/sway,taiyu-len/sway