str: Add missing include

<bit> should be included to use std::endian.

Change-Id: Id5d18ee1b700094ab18f6d9909056ed7ea4d7477
Signed-off-by: Peter Foley <pefoley@google.com>
diff --git a/include/stdplus/str/buf.hpp b/include/stdplus/str/buf.hpp
index 1870619..eed6314 100644
--- a/include/stdplus/str/buf.hpp
+++ b/include/stdplus/str/buf.hpp
@@ -1,4 +1,6 @@
 #pragma once
+
+#include <bit>
 #include <cstdint>
 #include <limits>
 #include <memory>