Revert "Move constant_flow.h into the main library"

This reverts commit fd78c34e23c95a49b0c0790b714aee7cd85335c9.

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