libspl: fix warning about missing spl_pagesize declaration

[ericonr]

Though it's unlikely anyone will alter its signature, avoids any
possible type mismatch.

Motivation and Context

Noticed warning while building, but it's always been there I think.

Description

How Has This Been Tested?

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Performance enhancement (non-breaking change which improves efficiency)
• Code cleanup (non-breaking change which makes code smaller or more readable)
• Breaking change (fix or feature that would cause existing functionality to change)
• Library ABI change (libzfs, libzfs_core, libnvpair, libuutil and libzfsbootenv)
• Documentation (a change to man pages or other documentation)

Checklist:

• My code follows the OpenZFS code style requirements.
• I have updated the documentation accordingly.
• I have read the contributing document.
• I have added tests to cover my changes.
• I have run the ZFS Test Suite with this change applied.
• All commit messages are properly formatted and contain Signed-off-by.

[behlendorf]

Makes sense, interestingly I don't think I've seen this warning. What environment are you seeing this in (distribution, compiler, etc)?

[ericonr]

Building zfs 2.1.1 on Void Linux, using a snapshot of GCC 10.2.x branch (as snapshotted by alpine). musl libc too, but I don't think it matters here. No CFLAGS I can think of that'd affect the results here.

[tonynguien]

Looks good.