spl: fix compilation without HAVE_BACKTRACE

[mmatuska]

Motivation and Context

Without HAVE_BACKTRACE, the compilation of lib/libspl/backtrace.c fails on undefined __maybe_unused macro on both Linux and FreeBSD. This macro is defined in spl/sys/debug.h for both Linux and FreeBSD platforms.

Description

Add the sys/debug.h header to lib/libspl/backtrace.c

How Has This Been Tested?

Tested build on Linux, FreeBSD and FreeBSD base (bundled version of OpenZFS)

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.