patch 8.1.0863: cannot see what signal caused a job to end

Problem:    Cannot see what signal caused a job to end.
Solution:   Add "termsig" to job_info(). (Ozaki Kiichi, closes #3786)
diff --git a/src/structs.h b/src/structs.h
index 459103d..ddc56bc 100644
--- a/src/structs.h
+++ b/src/structs.h
@@ -1550,7 +1550,10 @@
     char_u	*jv_tty_in;	/* controlling tty input, allocated */
     char_u	*jv_tty_out;	/* controlling tty output, allocated */
     jobstatus_T	jv_status;
-    char_u	*jv_stoponexit; /* allocated */
+    char_u	*jv_stoponexit;	/* allocated */
+#ifdef UNIX
+    char_u	*jv_termsig;	/* allocated */
+#endif
     int		jv_exitval;
     char_u	*jv_exit_cb;	/* allocated */
     partial_T	*jv_exit_partial;