libwebsockets: enable external poll support

This was removed and then re-added, but defaulted off upstream.  It's
still listed as "not recommended" but still absolutely essential for
many apps for performance.

Fixes: eclipse/mosquitto#2284

Signed-off-by: Karl Palsson <karlp@etactica.com>

libs/libwebsockets/Makefile

@@ -31,6 +31,7 @@ include $(INCLUDE_DIR)/cmake.mk
 CMAKE_OPTIONS += -DLWS_IPV6=$(if $(CONFIG_IPV6),ON,OFF)
 CMAKE_OPTIONS += -DISABLE_WERROR=ON
 CMAKE_OPTIONS += -DCMAKE_BUILD_TYPE=Release
+CMAKE_OPTIONS += -DLWS_WITH_EXTERNAL_POLL=ON
 CMAKE_OPTIONS += -DLWS_WITH_ULOOP=ON
 
 # turn off all test apps