net: http_client: add missing kernel.h include statement

There is following error when compiling applications using http_client:

  /zephyr/include/net/http_client.h:157:24: error: field 'work' has \
  incomplete type
    157 |  struct k_delayed_work work;
        |                        ^~~~
  /zephyr/include/net/http_client.h:177:2: error: unknown type name \
  'k_timeout_t'
    177 |  k_timeout_t timeout;
        |  ^~~~~~~~~~~

Fix that by adding missing kernel.h include statement in http_client.h.

Signed-off-by: Marcin Niestroj <m.niestroj@grinn-global.com>