patch 8.2.3941: SIGTSTP is not handled

Problem:    SIGTSTP is not handled.
Solution:   Handle SIGTSTP like pressing CTRL-Z. (closes #9422)
diff --git a/src/proto/ex_docmd.pro b/src/proto/ex_docmd.pro
index d88b8a3..0fed2f0 100644
--- a/src/proto/ex_docmd.pro
+++ b/src/proto/ex_docmd.pro
@@ -40,6 +40,7 @@
 void ex_quit(exarg_T *eap);
 void tabpage_close(int forceit);
 void tabpage_close_other(tabpage_T *tp, int forceit);
+void ex_stop(exarg_T *eap);
 void handle_drop(int filec, char_u **filev, int split, void (*callback)(void *), void *cookie);
 void handle_any_postponed_drop(void);
 void ex_splitview(exarg_T *eap);