cmd/clangd: add _GNU_SOURCE

Add -D_GNU_SOURCE so that clangd doesn't complain about O_PATH.

Signed-off-by: Maciej Borzecki <maciej.borzecki@canonical.com>

cmd/.clangd

@@ -1,2 +1,8 @@
+# -*- Mode: yaml -*-
+
 CompileFlags:
-  Add: [-I/usr/include/glib-2.0, -Wall, -Wextra, -Wmissing-prototypes, -Wstrict-prototypes, -Wno-missing-field-initializers, -Wno-unused-parameter]
+  Add: [-I/usr/include/glib-2.0,
+        -Wall, -Wextra, -Wmissing-prototypes,
+        -Wstrict-prototypes, -Wno-missing-field-initializers,
+        -Wno-unused-parameter,
+        -D_GNU_SOURCE]