helpers: fixup bad fadvise check

Signed-off-by: Jens Axboe <axboe@kernel.dk>
diff --git a/helpers.c b/helpers.c
index dce967d..4342b2d 100644
--- a/helpers.c
+++ b/helpers.c
@@ -32,7 +32,7 @@
 }
 #endif
 
-#ifndef CONFIG_FADVISE
+#ifndef CONFIG_POSIX_FADVISE
 int posix_fadvise(int fd, off_t offset, off_t len, int advice)
 {
 	return 0;