Add explicit #includes for used isl features

llvm-svn: 236931
diff --git a/polly/lib/Transform/DeadCodeElimination.cpp b/polly/lib/Transform/DeadCodeElimination.cpp
index 8221f89..1d5ccbe 100644
--- a/polly/lib/Transform/DeadCodeElimination.cpp
+++ b/polly/lib/Transform/DeadCodeElimination.cpp
@@ -40,6 +40,7 @@
 #include "isl/map.h"
 #include "isl/set.h"
 #include "isl/union_map.h"
+#include "isl/union_set.h"
 
 using namespace llvm;
 using namespace polly;