patch 8.1.0087: v:shell_error is always zero when using terminal for "!cmd"

Problem:    v:shell_error is always zero when using terminal for "!cmd".
Solution:   Use "exitval" of terminal-job. (Ozaki Kiichi, closes #2994)
diff --git a/src/proto/terminal.pro b/src/proto/terminal.pro
index 25f9647..1031876 100644
--- a/src/proto/terminal.pro
+++ b/src/proto/terminal.pro
@@ -55,5 +55,6 @@
 void f_term_start(typval_T *argvars, typval_T *rettv);
 void f_term_wait(typval_T *argvars, typval_T *rettv);
 void term_send_eof(channel_T *ch);
+job_T *term_getjob(term_T *term);
 int terminal_enabled(void);
 /* vim: set ft=c : */