Explicitly link zfsd with libspl to avoid undefined references

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.

MFC after:	3 days