obmc-console: Extract definition of _GNU_SOURCE

Avoid triggering bugprone-reserved-identifier

Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
Change-Id: I8b4d40c92c1e9633bdb078c3f8d6d97ffb248cc2
diff --git a/meson.build b/meson.build
index 81f13a2..8787f2a 100644
--- a/meson.build
+++ b/meson.build
@@ -10,6 +10,8 @@
     meson_version: '>=0.63.0',
 )
 
+add_project_arguments('-D_GNU_SOURCE', language: 'c')
+
 systemdsystemunitdir = dependency('systemd').get_variable('systemdsystemunitdir')
 install_data('conf/obmc-console@.service.in',
              'conf/obmc-console@.socket.in',