libspl: Don't build tunables.c when bootstrapping

The upstream source (which we've currently hacked downstream to make it
compile, even though it won't work) assumes ELF for its hand-rolled
linker set implementation, and macOS's Mach-O is sufficiently different
for it to not even compile there. However, we don't actually need this
file at all, since we only use the bootstrap libspl for DTrace tools, so
just add it to the set of files we don't build when bootstrapping.

Fixes: df58e8b1506f ("zfs: merge openzfs/zfs@8302b6e32")