Add one use of ATTRIBUTE_UNUSED

One spot needed ATTRIBUTE_UNUSED to cope with the new warnings.

The case in inflow.c is just a mass of ifdefs; and while the only use
of "result" is guarded by "#if 0", I thought it simplest to leave it
all in place.

2016-07-14  Tom Tromey  <tom@tromey.com>

	* inflow.c (child_terminal_ours_1): Use ATTRIBUTE_UNUSED.