Add tsdb/tsdb to .gitignore
Add tsdb/tsdb to .gitignore

This is built this way by the Makefile. One might ask if that's the
correct location for the `tsdb` tool or if it should be placed
top-level together with `prometheus` and `promtool`. But for now, it
should be excluded from Git like the latter two.

Signed-off-by: beorn7 <91e32ae1d26bd312b370cf8378a5a83accca90a0@grafana.com>

GoogleCloudPlatform/prometheus,keegancsmith/prometheus,GoogleCloudPlatform/prometheus,GoogleCloudPlatform/prometheus,kangwoo/prometheus,GoogleCloudPlatform/prometheus,Gouthamve/prometheus,keegancsmith/prometheus,alileza/prometheus,prometheus/prometheus,kangwoo/prometheus,keegancsmith/prometheus,kangwoo/prometheus,alileza/prometheus,kangwoo/prometheus,prometheus/prometheus,alileza/prometheus,Gouthamve/prometheus,GoogleCloudPlatform/prometheus,alileza/prometheus,keegancsmith/prometheus,alileza/prometheus,prometheus/prometheus,prometheus/prometheus,Gouthamve/prometheus,keegancsmith/prometheus,prometheus/prometheus,Gouthamve/prometheus