print: Add missing include guard

Change-Id: I78f872afb21a17e697da2900d83624fcdea4b8f7
Signed-off-by: Vlad Sytchenko <vsytch@google.com>
diff --git a/include/stdplus/print.hpp b/include/stdplus/print.hpp
index bc49570..8a61e40 100644
--- a/include/stdplus/print.hpp
+++ b/include/stdplus/print.hpp
@@ -1,3 +1,4 @@
+#pragma once
 #include <stdplus/str/buf.hpp>
 
 #include <format>