Wire up _weak posix_fadvise()

The commit adding Android support included a botched
variant of posix_fadvise().

Signed-off-by: Jens Axboe <axboe@kernel.dk>
diff --git a/helpers.c b/helpers.c
index 5be45cc..1b4e1d0 100644
--- a/helpers.c
+++ b/helpers.c
@@ -50,3 +50,10 @@
 	return -1;
 }
 #endif
+
+#ifndef FIO_HAVE_FADVISE
+int _weak posix_fadvise(int fd, off_t offset, off_t len, int advice)
+{
+	return 0;
+}
+#endif