patch 8.1.2094: the fileio.c file is too big

Problem:    The fileio.c file is too big.
Solution:   Move buf_write() to bufwrite.c. (Yegappan Lakshmanan,
            closes #4990)
diff --git a/src/Make_morph.mak b/src/Make_morph.mak
index f0d9cf8..247a578 100644
--- a/src/Make_morph.mak
+++ b/src/Make_morph.mak
@@ -29,6 +29,7 @@
 	autocmd.c						\
 	blowfish.c						\
 	buffer.c						\
+	bufwrite.c						\
 	change.c						\
 	charset.c						\
 	cmdexpand.c						\