wire: export w1_touch_bit

The w1_ds28e17 driver from the next part of this patch needs to emit
single-bit read timeslots to the DS28E17. The w1 subsystem already
has this function but it is not exported outside drivers/w1/w1_io.c

This subpatch exports the w1_touch_bit symbol with EXPORT_SYMBOL_GPL,
same as the other exported symbols in drivers/w1/w1_io.c

May be also useful later for writing drivers for other Onewire chips
which do single-bit communication.

Signed-off-by: Jan Kandziora <jjj@gmx.de>
Acked-by: Evgeniy Polyakov <zbr@ioremap.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>