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_cyg_ming.mak b/src/Make_cyg_ming.mak
index 01f6233..503761c 100644
--- a/src/Make_cyg_ming.mak
+++ b/src/Make_cyg_ming.mak
@@ -709,6 +709,7 @@
 	$(OUTDIR)/blob.o \
 	$(OUTDIR)/blowfish.o \
 	$(OUTDIR)/buffer.o \
+	$(OUTDIR)/bufwrite.o \
 	$(OUTDIR)/change.o \
 	$(OUTDIR)/charset.o \
 	$(OUTDIR)/cmdexpand.o \