Move constant_flow.h into the main library

Signed-off-by: Dave Rodgman <dave.rodgman@arm.com>
diff --git a/library/constant_time.c b/library/constant_time.c
index d4bd331..61fb094 100644
--- a/library/constant_time.c
+++ b/library/constant_time.c
@@ -30,7 +30,7 @@
 #include "mbedtls/error.h"
 #include "mbedtls/platform_util.h"
 
-#include "../tests/include/test/constant_flow.h"
+#include "constant_flow.h"
 
 #include <string.h>