AIX testsuite failure due to missing template instantiation

[Benjamin Kosnik]

>FAIL: 27_io/basic_filebuf/underflow/10096.cc (test for excess errors)
>Excess errors:
>ld: 0711-317 ERROR: Undefined symbol: std::basic_filebuf<char, MyTraits>::_S_pback_size

Should be fixed now, thanks.