dm verity: Fix compilation warning

For the case !CONFIG_DM_VERITY_VERIFY_ROOTHASH_SIG, declare the
functions verity_verify_root_hash(), verity_verify_is_sig_opt_arg(),
verity_verify_sig_parse_opt_args() and verity_verify_sig_opts_cleanup()
as inline to avoid a "no previous prototype for xxx" compilation
warning when compiling with W=1.

Signed-off-by: Damien Le Moal <damien.lemoal@wdc.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>