Explicitly specify the path to wpp.h so we don't need to change the
include path, to avoid conflicts with multiple y.tab.h files.

diff --git a/tools/widl/parser.l b/tools/widl/parser.l
index 47a4748..0ff67cf 100644
--- a/tools/widl/parser.l
+++ b/tools/widl/parser.l
@@ -42,7 +42,7 @@
 #include "widl.h"
 #include "utils.h"
 #include "parser.h"
-#include "wpp.h"
+#include "../wpp/wpp.h"
 
 #include "y.tab.h"