gimple_iter_impl ctor takes a gimple_seq, rather than a gimple

Fix the build of libir-gcc-plugin.cc on GCC 4.7 and earlier.

Note: GCC 4.8 onwards eliminated the struct gimple_seq_node_d node within
a gimple_seq in favor of putting the next/prev fields directly within
gimple_statement_base.

The specific commit was r187053 aka e3a19533c3ba16854e1dc1bb2ac7ce197883e38f.