net: websocket: Make sure the mbedtls_sha1 function is build in

In the header the websocket protocol needs a SHA1 hash. This is
implemented using the mbedtls_sha1 function. Select the option
MBEDTLS_MAC_SHA1_ENABLED from the Kconfig of websocket to ensure this
function is build in.

Signed-off-by: Sjors Hettinga <s.a.hettinga@gmail.com>