Omit spurious include of <sys/isa_defs.h> in dtrace tools build.

Fixes dtrace tools cross-build from architectures with no dtrace
support:

#  compile  libctf/ctf_create.lo
[...]
/export/anoncvs/src/tools/libctf/../../external/cddl/osnet/sys/sys/isa_defs.h:50:2: error: #error "architecture not supported"
#error "architecture not supported"
  ^

(riastradh)