THRIFT-901. hs: Allow the bindings to compile without -fglasgow-exts and with -Wall -Werror

Forgot a small chunk of the previous patch.

Patch: Christian Lavoie

git-svn-id: https://svn.apache.org/repos/asf/incubator/thrift/trunk@999019 13f79535-47bb-0310-9956-ffa450edef68
diff --git a/compiler/cpp/src/generate/t_hs_generator.cc b/compiler/cpp/src/generate/t_hs_generator.cc
index 4ddd8cd..7b178df 100644
--- a/compiler/cpp/src/generate/t_hs_generator.cc
+++ b/compiler/cpp/src/generate/t_hs_generator.cc
@@ -574,7 +574,7 @@
   // read
   indent(out) << "read_"<<sname<<" iprot = do" << endl;
   indent_up();
-  indent(out) << "readStructBegin iprot" << endl;
+  indent(out) << "_ <- readStructBegin iprot" << endl;
   indent(out) << "rec <- read_"<<sname<<"_fields iprot ("<<sname<<"{";
   bool first = true;
   for (f_iter = fields.begin(); f_iter != fields.end(); ++f_iter) {