meson: Clarify license

SPDX is the standard recommended by Meson.
3 jobs for master in 47 minutes and 9 seconds (queued for 1 second)
Status Job ID Name Coverage
  Check
passed #1207596
check

00:01:54

 
  Test
passed #1207597
flatpak
flatpak

00:41:49

 
  Deploy
passed #1207598
nightly

00:03:25