WPS: Add multi_ap_subelem to wps_build_wfa_ext()

WPS: Add multi_ap_subelem to wps_build_wfa_ext()

The Multi-AP specification adds a new subelement to the WFA extension
element in the WPS exchange. Add an additional parameter to
wps_build_wfa_ext() to add this subelement. The subelement is only added
if the parameter is nonzero. Note that we don't reuse the existing
MULTI_AP_SUB_ELEM_TYPE definition here, but rather define a new
WFA_ELEM_MULTI_AP, to make sure the enum of WFA subelement types for WPS
vendor extension remains complete.

For now, all callers set the multi_ap_subelem parameter to 0.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>