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 #8935156
check-commit

00:00:09

 
  Prep
passed #8935157
debian:buster@container-prep

00:00:15

 
  Build
passed #8935158
build-native

00:00:46