Add missing pragma once to top of header file

While starting to work on code that uses NamesValues I ran into a class
redefinition compile failure due to this missing line.

Change-Id: Ic8fe28a47af4cdefe277f2c03e78a26ab3b83c1b
Signed-off-by: Brandon Wyman <bjwyman@gmail.com>
diff --git a/names_values.hpp b/names_values.hpp
index 39b5739..1e03d51 100644
--- a/names_values.hpp
+++ b/names_values.hpp
@@ -1,3 +1,4 @@
+#pragma once
 #include <sstream>
 #include <string>