patch 8.0.1102: terminal window does not use Normal colors

Problem:    Terminal window does not use Normal colors.
Solution:   For the GUI and when 'termguicolors' is enabled, use the actual
            foreground and background colors for the terminal.  (Yasuhiro
            Matsumoto, closes #2067)
            Use the "Terminal" highlight group if defined.
diff --git a/src/proto/syntax.pro b/src/proto/syntax.pro
index 7247c51..0f64cea 100644
--- a/src/proto/syntax.pro
+++ b/src/proto/syntax.pro
@@ -52,6 +52,7 @@
 int syn_check_group(char_u *pp, int len);
 int syn_id2attr(int hl_id);
 int syn_id2colors(int hl_id, guicolor_T *fgp, guicolor_T *bgp);
+void syn_id2cterm_bg(int hl_id, int *fgp, int *bgp);
 int syn_get_final_id(int hl_id);
 void highlight_gui_started(void);
 int highlight_changed(void);