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/proto.h b/src/proto.h
index 4c419c6..625e8aa 100644
--- a/src/proto.h
+++ b/src/proto.h
@@ -64,6 +64,7 @@
 # include "arglist.pro"
 # include "autocmd.pro"
 # include "buffer.pro"
+# include "bufwrite.pro"
 # include "change.pro"
 # include "charset.pro"
 # include "cmdexpand.pro"