meson.build: install the protocol .txt files in doc

This now matches the autotools installation

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
8 jobs for wip/meson-fixes in 1 minute and 34 seconds
Status Name Job ID Coverage
  Prep
passed container-prep #7868282

00:00:26

 
  Check
passed check formatting #7868283

00:00:09

 
  Build
passed autotools: [] #7868286

00:00:43

passed autotools: [--enable-legacy] #7868287

00:00:43

passed meson: [] #7868284

00:00:13

passed meson: [-Dlegacy=true] #7868285

00:00:13

 
  Test
passed check evdev keysyms #7868289

00:00:08

passed meson from tarball #7868288

00:00:14