patch 8.1.0230: directly checking 'buftype' value

Problem:    Directly checking 'buftype' value.
Solution:   Add the bt_normal() function. (Yegappan Lakshmanan)
diff --git a/src/proto/buffer.pro b/src/proto/buffer.pro
index 9e63fb6..6820d97 100644
--- a/src/proto/buffer.pro
+++ b/src/proto/buffer.pro
@@ -56,6 +56,7 @@
 void do_modelines(int flags);
 int read_viminfo_bufferlist(vir_T *virp, int writing);
 void write_viminfo_bufferlist(FILE *fp);
+int bt_normal(buf_T *buf);
 int bt_quickfix(buf_T *buf);
 int bt_terminal(buf_T *buf);
 int bt_help(buf_T *buf);