cmd: bcb: Fix bcb compilation when CONFIG_CMD_BCB=n

cmd: bcb: Fix bcb compilation when CONFIG_CMD_BCB=n

commit dfeb4f0d7935 ("cmd: bcb: extend BCB C API to allow read/write the fields")
introduced the bcb_get() function.

When CONFIG_CMD_BCB=n, that function is stubbed.
The stubbed function has a wrong prototype: value_size arg is missing.

Add the missing argument to fix build when CONFIG_CMD_BCB=n.

Fixes: dfeb4f0d7935 ("cmd: bcb: extend BCB C API to allow read/write the fields")
Signed-off-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>
Reviewed-by: Guillaume La Roque <glaroque@baylibre.com>
Reviewed-by: Dmitrii Merkurev <dimorinny@google.com>
Reviewed-by: Julien Masson <jmasson@baylibre.com>