WPS: Add HTTP client module to clean up code

Instead of implementing HTTP client functionality inside
wps_upnp_event.c, use a generic HTTP client module to do this. The HTTP
client code can now be shared more easily for other purposes, too.

wpa_supplicant/Makefile

@@ -510,6 +510,7 @@ OBJS += ../src/wps/wps_upnp_ssdp.o
 OBJS += ../src/wps/wps_upnp_web.o
 OBJS += ../src/wps/wps_upnp_event.o
 OBJS += ../src/wps/httpread.o
+OBJS += ../src/wps/http_client.o
 endif
 
 endif