patch 8.0.1610: cannot build without GUI

Problem:    Cannot build without GUI.
Solution:   Add #ifdef.
diff --git a/src/terminal.c b/src/terminal.c
index 89a3541..bff83d2 100644
--- a/src/terminal.c
+++ b/src/terminal.c
@@ -2735,6 +2735,7 @@
     }
 }
 
+#if defined(FEAT_GUI)
     static void
 update_system_term(term_T *term)
 {
@@ -2777,6 +2778,7 @@
     term->tl_dirty_row_end = 0;
     update_cursor(term, TRUE);
 }
+#endif
 
 /*
  * Called to update a window that contains an active terminal.