patch 8.2.4594: need to write script to a file to be able to source them

Problem:    Need to write script to a file to be able to source them.
Solution:   Make ":source" use lines from the current buffer. (Yegappan
            Lakshmanan et al., closes #9967)
diff --git a/src/proto/scriptfile.pro b/src/proto/scriptfile.pro
index 477dc3e..d0ebdda 100644
--- a/src/proto/scriptfile.pro
+++ b/src/proto/scriptfile.pro
@@ -42,4 +42,5 @@
 char_u *may_prefix_autoload(char_u *name);
 char_u *autoload_name(char_u *name);
 int script_autoload(char_u *name, int reload);
+int sourcing_a_script(exarg_T *eap);
 /* vim: set ft=c : */