patch 8.1.1135: build failure for small version

Problem:    Build failure for small version. (Tony Mechelynck)
Solution:   Add #ifdef.
diff --git a/src/buffer.c b/src/buffer.c
index 187a8a6..8691500 100644
--- a/src/buffer.c
+++ b/src/buffer.c
@@ -1853,7 +1853,9 @@
 	&& curbuf->b_ffname == NULL
 	&& curbuf->b_nwindows <= 1
 	&& (curbuf->b_ml.ml_mfp == NULL || BUFEMPTY())
+#if defined(FEAT_QUICKFIX)
 	&& !bt_quickfix(curbuf)
+#endif
 	&& !curbufIsChanged());
 }