meson: Do not add compiler flags if 'plain' buildtype is used

That is how the 'plain' buildtype is meant in meson.

GNOME/gnome-shell!464
4 jobs for !464 with debug-builds in 4 minutes and 53 seconds
detached
Status Job ID Name Coverage
  Review
passed #312227
check_commit_log

00:00:40

 
  Build
passed #312228
build

00:02:05

 
  Test
passed #312229
test

00:02:03

passed #312230
test-pot

00:02:07