Explicitly link zfsd with libspl to avoid undefined references

Because lld 13.0.0 is more strict about undefined references when
linking to shared libraries, it produces the following errors for zfsd:

ld: error: /home/dim/obj/home/dim/src/llvm-13-update/amd64.amd64/tmp/usr/lib/libzfs_core.so: undefined reference to libspl_assertf [--no-allow-shlib-undefined]
ld: error: /home/dim/obj/home/dim/src/llvm-13-update/amd64.amd64/tmp/usr/lib/libnvpair.so: undefined reference to libspl_assertf [--no-allow-shlib-undefined]
ld: error: /home/dim/obj/home/dim/src/llvm-13-update/amd64.amd64/tmp/usr/lib/libavl.so: undefined reference to libspl_assertf [--no-allow-shlib-undefined]

• [zfsd.full] Error code 1

Fix this by adding libspl (where libspl_assertf lives) to zfsd's LIBADD.

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