Fix doxygen error

Signed-off-by: Dave Rodgman <dave.rodgman@arm.com>
diff --git a/library/constant_flow.h b/library/constant_flow.h
index ff464e6..9554cfc 100644
--- a/library/constant_flow.h
+++ b/library/constant_flow.h
@@ -26,7 +26,7 @@
 
 #include "mbedtls/build_info.h"
 
-/*
+/**
  * This file defines the two macros
  *
  *  #define TEST_CF_SECRET(ptr, size)