AIX testsuite failure due to missing template instantiation

[David Edelsohn]

	I believe that the new test underflow/10096.cc is failing due to a
missing template instantiation:

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

David