patch 8.1.1767: FEAT_SESSION defined separately

Problem:    FEAT_SESSION defined separately.
Solution:   Make FEAT_SESSION depend on FEAT_EVAL.
diff --git a/src/feature.h b/src/feature.h
index 74ea8d5..e46947d 100644
--- a/src/feature.h
+++ b/src/feature.h
@@ -559,9 +559,9 @@
 
 /*
  * +mksession		":mksession" command.
- *			Requires +windows and +vertsplit.
+ *			fully depends on +eval
  */
-#if defined(FEAT_NORMAL)
+#if defined(FEAT_EVAL)
 # define FEAT_SESSION
 #endif