tools/wlanstats: fix -Wunused-but-set-variable

The "allnodes" code is currently #if 0 so also #if 0 the setting of the
command line option and variable in order to keep wlanstats compiling.

Sponsored by: The FreeBSD Foundation

(cherry picked from commit 530c13c5401c1f29f8325cf851d96778852a1ba3)