IYWU: include necessary header for constant_fold()

Signed-off-by: Henner Zeller <h.zeller@acm.org>

Author: hzeller

src/syn/test/opt_test.cc

@@ -4,6 +4,7 @@
 #include <memory>
 #include <sstream>
 
+#include "flow/constant_fold.h"
 #include "gtest/gtest.h"
 #include "syn/ir/Bundle.h"
 #include "syn/ir/Const.h"
@@ -24,8 +25,6 @@ int main(int argc, char** argv)
 
 namespace syn {
 
-void constantFold(Graph& g);
-
 TEST(OptTest, AndZero)
 {
   Graph g;