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/Make_morph.mak b/src/Make_morph.mak
index 8d93d7e..f292934 100644
--- a/src/Make_morph.mak
+++ b/src/Make_morph.mak
@@ -75,6 +75,7 @@
 	regexp.c						\
 	screen.c						\
 	search.c						\
+	session.c						\
 	sha256.c						\
 	sign.c							\
 	spell.c							\