zbeacon.c: Remove unused variable in s_get_interface (mingw)

Related issues: #564

~~~~~
zbeacon.c: In function 's_get_interface':
zbeacon.c:578:16: error: variable 'friendlyLength' set but not used [-Werror=unused-but-set-variable]
         size_t friendlyLength = 0;
                ^
~~~~~