plperl: Package-qualify _TD

Failing to do so causes trigger invocation to fail when they are nested
within a function invocation that changes the current package.

Backpatch to 9.1; previous releases used a different method to obtain
_TD.  Per bug report from Mark Murawski (bug #6511)

Author: Alex Hunsaker

src/pl/plperl/plperl.c

@@ -2062,7 +2062,7 @@ plperl_call_perl_trigger_func(plperl_proc_desc *desc, FunctionCallInfo fcinfo,
 	ENTER;
 	SAVETMPS;
 
-	TDsv = get_sv("_TD", 0);
+	TDsv = get_sv("main::_TD", 0);
 	if (!TDsv)
 		elog(ERROR, "couldn't fetch $_TD");