Fix trivial -Wundef warnings

* See [Feature #17752]

Co-authored-by: xtkoba (Tee KOBAYASHI) <xtkoba+ruby@gmail.com>

1 files changed, 1 insertions, 1 deletions

diff --git a/vm.c b/vm.c
index 8c62d7dd91..ea8b21e7a5 100644
--- a/vm.c
+++ b/vm.c
@@ -3262,7 +3262,7 @@ static VALUE
 nsdr(VALUE self)
 {
     VALUE ary = rb_ary_new();
-#if HAVE_BACKTRACE
+#ifdef HAVE_BACKTRACE
 #include <execinfo.h>
 #define MAX_NATIVE_TRACE 1024
     static void *trace[MAX_NATIVE_TRACE];