libspl: fix warning about missing spl_pagesize declaration
Though it's unlikely anyone will alter its signature, avoids any
possible type mismatch.
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Tony Nguyen <tony.nguyen@delphix.com>
Signed-off-by: Érico Nogueira <erico.erc@gmail.com>
Closes #12567