tools: avoid implicit fallthrough in fdt_add_pubkey

tools: avoid implicit fallthrough in fdt_add_pubkey

When building with -Wimplicit-fallthrough we get a warning

    tools/fdt_add_pubkey.c:52:25: warning:
    this statement may fall through [-Wimplicit-fallthrough=]
       52 |                         print_help();
          |

Explicitly declare which functions don't return.

Fixes: 30238e99619c ("tools: add fdt_add_pubkey")
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Simon Glass <sjg@chromium.org>