patch 8.1.1766: code for writing session file is spread out

Problem:    Code for writing session file is spread out.
Solution:   Put it in one file. (Yegappan Lakshmanan, closes #4728)
diff --git a/src/proto.h b/src/proto.h
index cc1c8fb..6415112 100644
--- a/src/proto.h
+++ b/src/proto.h
@@ -183,6 +183,9 @@
 # endif
 # include "regexp.pro"
 # include "screen.pro"
+# ifdef FEAT_SESSION
+#  include "session.pro"
+# endif
 # if defined(FEAT_CRYPT) || defined(FEAT_PERSISTENT_UNDO)
 #  include "sha256.pro"
 # endif