hostapd: declare struct wpa_bss early

hostapd: declare struct wpa_bss early

wps_supplicant.h assumes that 'struct wpa_bss' is forward declared if
CONFIG_WPS is not defined.  With the later inclusion of
600-ubus_support, the issue manifests in warnings like these:

wps_supplicant.h:113:15: warning: 'struct wpa_bss' declared inside parameter list will not be visible outside of this definition or declaration
        struct wpa_bss *bss)
               ^~~~~~~

This patch forward declares 'struct wpa_bss' regardless.

Signed-off-by: Leon M. George <leon@georgemail.eu>
[commit message facelift]
Signed-off-by: Petr Štetiar <ynezz@true.cz>
(cherry picked from commit f974f8213b94578581b35e6b3f8fb1fd5a35f753)