meson: only require cpp for tests

Signed-off-by: James Hilliard <james.hilliard1@gmail.com>
3 jobs for test-cpp in 1 minute and 11 seconds (queued for 12 seconds)
Status Job ID Name Coverage
  Review
passed check-commit #8935156

00:00:09

 
  Prep
passed debian:buster@container-prep #8935157

00:00:15

 
  Build
passed build-native #8935158

00:00:46