readmmap.c: fix building with MUSL libc

[zdykstra]

Motivation and Context

Fix building readmmap.c with MUSL libc.

Description

glibc includes sys/types.h from stdlib.h. This is not the case for MUSL, so explicitly include it.

How Has This Been Tested?

Tested using Void's cross-compiling infrastructure - https://github.com/void-linux/void-packages/pull/45365/checks

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]

Thanks for testing with MUSL libc. Out of curiosity was this the only build issue you ran in to with the 2.2.0 release candidates? I'd like to include any additional minor fixes in the next 2.2.0-rc4 tag if possible.

[zdykstra]

That was the only issue I hit when building 2.2.0-rc3 with MUSL - though I haven't actually tested the build product.