iflib: Cast the result of iflib_netmap_txq_init() to void.

This fixes a warning from GCC for kernels without netmap since the
return value is never used.

Reviewed by: vmaffione, erj
Differential Revision: https://reviews.freebsd.org/D28598

(cherry picked from commit 2ccf971ace0bf23a9b27ec886a7f013c47661052)