patch 8.0.0366: build fails with tiny features

Problem:    Build fails with tiny features.
Solution:   Add #ifdef.
diff --git a/src/buffer.c b/src/buffer.c
index 73af50a..6edb48a 100644
--- a/src/buffer.c
+++ b/src/buffer.c
@@ -885,8 +885,10 @@
     di->di_tv.v_lock = VAR_FIXED;
     di->di_tv.vval.v_number = 0;
 
+#ifdef FEAT_EVAL
     STRCPY(buf->b_ct_di.di_key, "changedtick");
     (void)dict_add(buf->b_vars, di);
+#endif
 }
 
 /*