tools: iio: iio_generic_buffer: fix argument '?' option

When help ('?') option is passed to the command, the help text
printed but not from '?' switch case of getopt_long() but as a
invalid argument as below. Fix this by adding '?' to opt_String
of getopt_long().

root@am437x-evm:~# ./iio_generic_buffer -?
./iio_generic_buffer: invalid option -- '?'
Usage: generic_buffer [options]...
Capture, convert and output data from IIO device buffer
  -a         Auto-activate all available channels
...

Signed-off-by: Mugunthan V N <mugunthanvnm@ti.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>