Added missing bind_dev getter and setter.

[WatcherOfTheSkies]

PLEASE NOTE the following text from the iperf3 license. Submitting a
pull request to the iperf3 repository constitutes "[making]
Enhancements available...publicly":

You are under no obligation whatsoever to provide any bug fixes, patches, or
upgrades to the features, functionality or performance of the source code
("Enhancements") to anyone; however, if you choose to make your Enhancements
available either publicly, or directly to Lawrence Berkeley National
Laboratory, without imposing a separate written license agreement for such
Enhancements, then you hereby grant the following license: a non-exclusive,
royalty-free perpetual license to install, use, modify, prepare derivative
works, incorporate into other computer software, distribute, and sublicense
such enhancements or derivative works thereof, in binary and source code form.

The complete iperf3 license is available in the LICENSE file in the
top directory of the iperf3 source tree.

• Applies to master (and was meant for V3.11/V3.12)

• Issues fixed (if any): Allows iperf API users to get/set the bind device. e.g. wwan0

• Brief description of code changes (suitable for use as a commit message):
  Added missing bind_dev getter and setter.

[WatcherOfTheSkies]

Maybe the setter should have been:
const char *bnd_dev
(sorry!)

[bmah888]

Thanks for the PR! This fixes a buglet that I introduced in #1097 by not adding the proper function declarations to src/iperf_api.h.

[bmah888]

Looks good, thanks! It'd also be good to get the libiperf.3 manual page in sync with respect to const correctness, but there are actually several places where we need to fix this.