encoding: add missing include in nfd-constants.hpp

refs #2091

Change-Id: I0319506eac91b5a97a0a882dc95473a4bb56eb0a
diff --git a/tests/unit-tests/encoding/nfd-constants.cpp b/tests/unit-tests/encoding/nfd-constants.cpp
new file mode 100644
index 0000000..537699e
--- /dev/null
+++ b/tests/unit-tests/encoding/nfd-constants.cpp
@@ -0,0 +1,4 @@
+#include "encoding/nfd-constants.hpp"
+
+// This file is intentionally empty.
+// See Bug 2091.