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/term.c b/src/term.c
index fa0be05..26527fb 100644
--- a/src/term.c
+++ b/src/term.c
@@ -6399,4 +6399,14 @@
 
     return INVALCOLOR;
 }
+
+    guicolor_T
+gui_get_rgb_color_cmn(int r, int g, int b)
+{
+    guicolor_T  color = RGB(r, g, b);
+
+    if (color > 0xffffff)
+	return INVALCOLOR;
+    return color;
+}
 #endif