net: lib: http: fix CONFIG_HTTPS for client use

Fix build break when enabling CONFIG_HTTPS w/o CONFIG_NET_APP_SERVER

This error can be seen when building sample/net/http_client like so:
$ cd samples/net/http_client
$ mkdir build && cd build
$ cmake -DBOARD=qemu_x86 -DCONF_FILE=prj_tls.conf ..
$ make run

In file included from
             /home/<user>/zephyr/include/net/http.h:11:0,
        from /home/<user>/zephyr/samples/net/http_client/src/main.c:19:
/home/<user>/zephyr/include/net/http_app.h:643:11: error: unknown type
name ‘net_app_cert_cb_t’
           net_app_cert_cb_t cert_cb,
           ^~~~~~~~~~~~~~~~~
CMakeFiles/app.dir/build.make:302: recipe for target
'CMakeFiles/app.dir/src/main.c.obj' failed

Signed-off-by: Michael Scott <michael@opensourcefoundries.com>