Fix surrounding macro
diff --git a/bn_mp_error_to_string.c b/bn_mp_error_to_string.c
index 274c425..0e2af44 100644
--- a/bn_mp_error_to_string.c
+++ b/bn_mp_error_to_string.c
@@ -1,5 +1,5 @@
#include "tommath_private.h"
-#ifdef BN_ERROR_C
+#ifdef BN_MP_ERROR_TO_STRING_C
/* LibTomMath, multiple-precision integer library -- Tom St Denis */
/* SPDX-License-Identifier: Unlicense */