patch 8.1.0997: using GUI colors in vim.exe when 'termguicolors' is off

Problem:    Using GUI colors in vim.exe when 'termguicolors' is off.
Solution:   Add condition for 'termguicolors' set. (Ken Takata, closes #4078)
diff --git a/src/os_win32.c b/src/os_win32.c
index df6f7db..876a7a4 100644
--- a/src/os_win32.c
+++ b/src/os_win32.c
@@ -7844,7 +7844,7 @@
 	return;
 
     id = syn_name2id((char_u *)"Normal");
-    if (id > 0)
+    if (id > 0 && p_tgc)
 	syn_id2colors(id, &fg, &bg);
     if (fg == INVALCOLOR)
     {