build: Refactor to ensure all headers run through compile

Change-Id: I06089ea54ace1acd79d4d12d9b7eb6cc01b38596
Signed-off-by: William A. Kennington III <wak@google.com>
diff --git a/src/flags.cpp b/src/flags.cpp
new file mode 100644
index 0000000..1a8d495
--- /dev/null
+++ b/src/flags.cpp
@@ -0,0 +1 @@
+#include <stdplus/flags.hpp>