ast: remove comment re. anonymous struct

C11 is not sufficient for this, needs `--ms-extensions` which we don't
want to enable.

Signed-off-by: Ran Benita <ran@unusedvar.com>

src/xkbcomp/ast.h

@@ -243,7 +243,6 @@ typedef struct {
 
 union ExprDef {
     ParseCommon common;
-    /* Maybe someday we can use C11 anonymous struct for ExprCommon here. */
     ExprCommon expr;
     ExprIdent ident;
     ExprString string;