#include <unistd.h> which is required for unlink().

diff --git a/tools/widl/parser.l b/tools/widl/parser.l
index 0ff67cf..d89e479 100644
--- a/tools/widl/parser.l
+++ b/tools/widl/parser.l
@@ -38,6 +38,7 @@
 #include <string.h>
 #include <ctype.h>
 #include <assert.h>
+#include <unistd.h>
 
 #include "widl.h"
 #include "utils.h"