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/proto.h b/src/proto.h
index 6415112..2257012 100644
--- a/src/proto.h
+++ b/src/proto.h
@@ -183,9 +183,7 @@
 # endif
 # include "regexp.pro"
 # include "screen.pro"
-# ifdef FEAT_SESSION
-#  include "session.pro"
-# endif
+# include "session.pro"
 # if defined(FEAT_CRYPT) || defined(FEAT_PERSISTENT_UNDO)
 #  include "sha256.pro"
 # endif