Don't overlink libmt consumers with libsbuf.

This change came in r281332 which was reducing overlinking in mt(1) but
currently mt(1) is linked with sbuf when it does not need it due to the
LDADD_mt+=${LDADD_sbuf}. Only libmt needs sbuf.

Add sbuf to _DP_mt so static linkage of libmt picks it up.

Sponsored by: EMC / Isilon Storage Division