patch 8.0.0755: terminal window does not have colors in the GUI

Problem:    Terminal window does not have colors in the GUI.
Solution:   Lookup the GUI color.
diff --git a/src/gui_w32.c b/src/gui_w32.c
index 6119025..a91e071 100644
--- a/src/gui_w32.c
+++ b/src/gui_w32.c
@@ -1597,6 +1597,12 @@
     return gui_get_color_cmn(name);
 }
 
+    guicolor_T
+gui_mch_get_rgb_color(int r, int g, int b)
+{
+    return gui_get_rgb_color_cmn(r, g, b);
+}
+
 /*
  * Return OK if the key with the termcap name "name" is supported.
  */