[LTO/gold] Fix workaround for old plugin-api.h in --wrap support

The workaround for older plugin-api.h in r327506 unfortunately
used another union member that is also fairly new and not available
in the plugin-api.h on some of the bots, leading to:
http://lab.llvm.org:8011/builders/clang-with-thin-lto-ubuntu/builds/9121/steps/build-stage2-LLVMgold.so/logs/stdio

Change to use a different member that we will definitely have (as it
is used elsewhere in gold-plugin.cpp already).

llvm-svn: 327509
diff --git a/llvm/tools/gold/gold-plugin.cpp b/llvm/tools/gold/gold-plugin.cpp
index d6eb7a2..ebd6260 100644
--- a/llvm/tools/gold/gold-plugin.cpp
+++ b/llvm/tools/gold/gold-plugin.cpp
@@ -384,7 +384,7 @@
       // required version, this should be changed to:
       // get_wrap_symbols = tv->tv_u.tv_get_wrap_symbols;
       get_wrap_symbols =
-          (ld_plugin_get_wrap_symbols)tv->tv_u.tv_register_new_input;
+          (ld_plugin_get_wrap_symbols)tv->tv_u.tv_message;
       break;
     default:
       break;