fix include problem
Integrating DataSketches Theta into Apache Impala and compile error：
theta_union.hpp:27:10: fatal error: theta_sketch.hpp: No such file or directory
#include <theta_sketch.hpp>
diff --git a/theta/include/theta_union.hpp b/theta/include/theta_union.hpp
index 7fa2095..6cf8ccc 100644
--- a/theta/include/theta_union.hpp
+++ b/theta/include/theta_union.hpp
@@ -24,7 +24,7 @@
#include <functional>
#include <climits>
-#include <theta_sketch.hpp>
+#include "theta_sketch.hpp"
namespace datasketches {
