meson: Fix building unit tests when protocol headers aren't in default include path

4 jobs for meson-unit-test-tweak in 6 minutes and 21 seconds (queued for 1 second)
Status Job ID Name Coverage
  Docker Image
passed #267626
debian-testing

00:00:36

 
  Build And Test
passed #267627
autotools-build-and-test

00:05:44

passed #267628
meson-build-and-test

00:01:33

 
  External
passed continuous-integration/appveyor/branch