staging: rtl8723bs: ignore unused wiphy_wowlan object warnings

staging: rtl8723bs: ignore unused wiphy_wowlan object warnings

The wake-on-wlan stub is unused when the device power management
functionality is disabled in the kernel, creating a warning when
building the driver.

drivers/staging/rtl8723bs/os_dep/ioctl_cfg80211.c:50:42: warning: ‘wowlan_stub’ defined but not used [-Wunused-const-variable=]

Add the __maybe_unused annotation to silence this warning.

Signed-off-by: Bryan Brattlof <hello@bryanbrattlof.com>
Link: https://lore.kernel.org/r/20210916231928.2021584-1-hello@bryanbrattlof.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>