meson: GNU/kFreeBSD has DRM/KMS and requires -D_GNU_SOURCE

This is a regression from the old autotools build system.

Acked-by: Eric Engestrom <eric.engestrom@intel.com>
Acked-by: Dylan Baker <dylan@pnwbakers.com>
10 jobs for !810 with kfreebsd in 18 seconds (queued for 1 second)
detached
Status Job ID Name Coverage
  Containers Build
canceled debian #388357

00:00:18

 
  Build+Test
skipped meson-clang #388359
skipped meson-clover #388363
skipped meson-main #388362
skipped meson-swr-glvnd #388358
skipped meson-vulkan #388364
skipped scons-llvm #388366
skipped scons-nollvm #388365
skipped scons-swr #388360
skipped scons-win64 #388361