compiler: fix quoting of //go:linkname in error message

    
    Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/195718

From-SVN: r275758

gcc/go/gofrontend/MERGE

-5af62eda697da21155091cf5375ed9edb4639b67
+722990deeede7801e4ed3ca5d53ce312a19fcd7a
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.

gcc/go/gofrontend/gogo.cc

@@ -2541,7 +2541,7 @@ Gogo::add_linkname(const std::string& go_name, bool is_exported,
     {
       if (ext_name.empty())
 	go_error_at(loc,
-		    ("//%<go:linkname%> missing external name "
+		    ("%<//go:linkname%> missing external name "
 		     "for declaration of %s"),
 		    go_name.c_str());
       else