Annotate pragma/err.invalidlibdep.ksh as EXFAIL.

The test creates a D library with a "depends_on library" pragma
referencing a non-existent library, and expects compilation to fail.
However, as far as I can tell, libdtrace is supposed simply abort
compilation of the library in this case, and continue. This behaviour
is desirable when adding libraries which depend on optional KLDs, for
example.

MFC after: 1 week