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/bufwrite.pro b/src/proto/bufwrite.pro
new file mode 100644
index 0000000..8d81548
--- /dev/null
+++ b/src/proto/bufwrite.pro
@@ -0,0 +1,4 @@
+/* bufwrite.c */
+int buf_write(buf_T *buf, char_u *fname, char_u *sfname, linenr_T start, linenr_T end, exarg_T *eap, int append, int forceit, int reset_changed, int filtering);
+/* vim: set ft=c : */
+