mgmt: hawkbit: move variable declaration out of if

move variable declaration out of if,
because it will be optimized away, when
optimisation is enabled and leads to
undefined and unwanted behavior.

In my case `send_buf`  of `http_client_req`
was put at the same address and the
http request was malformed.

Signed-off-by: Fin Maaß <f.maass@vogl-electronic.com>