Fix llabs() ambiguity

some older systems (e.g. OSX/PPC) fail to compile with
  error: call of overloaded 'llabs(long long int)' is ambiguous
  candidates are:
      long long int llabs(long long int)
      long long int __gnu_cxx::llabs(long long int)

libs/ardour/disk_reader.cc

@@ -645,7 +645,7 @@ DiskReader::internal_playback_seek (sampleoffset_t distance)
	boost::shared_ptr<ChannelList> c = channels.reader();
	for (chan = c->begin(); chan != c->end(); ++chan) {
		if (distance < 0) {
			off = 0 - (sampleoffset_t) (*chan)->rbuf->decrement_read_ptr (llabs (distance));
			off = 0 - (sampleoffset_t) (*chan)->rbuf->decrement_read_ptr (::llabs (distance));
		} else {
			off = (*chan)->rbuf->increment_read_ptr (distance);
		}
@@ -1109,7 +1109,7 @@ void
DiskReader::get_midi_playback (MidiBuffer& dst, samplepos_t start_sample, samplepos_t end_sample, MonitorState ms, BufferSet& scratch_bufs, double speed, samplecnt_t disk_samples_to_consume)
{
	MidiBuffer* target;
	samplepos_t nframes = llabs (end_sample - start_sample);
	samplepos_t nframes = ::llabs (end_sample - start_sample);

	assert (_midi_buf);