afb-websock: Fix a double free

The commit 0ebdff94d461abd4328cf45a6281c15139a045eb of
libmicrohttpd fixed a bug but introduced a double free
of hreq resource attached to the connection. That commit
is between versions 0.9.59 and 0.9.60 of libmicrohttpd.

Now, hreq is freed in 'end_handler' of afb-hsrv.c

Change-Id: Ia865d65c5f9ef5ce787eb6545802ac8ea37ae468
Signed-off-by: José Bollo <jose.bollo@iot.bzh>