Disable some benign warnings.

[yallop]

The build's currently rather noisy, due to lots of warnings. A small sample:

Warning 44: this open statement shadows the value identifier pred (which is later used)
File "lib/tar_mirage.ml", line 114, characters 6-1337:
Warning 44: this open statement shadows the value identifier succ (which is later used)
File "lib/tar_mirage.ml", line 130, characters 29-85:
Warning 44: this open statement shadows the value identifier mul (which is later used)
File "lib/tar_mirage.ml", line 131, characters 61-125:
Warning 44: this open statement shadows the value identifier to_int (which is later used)
File "lib/tar_mirage.ml", line 131, characters 61-125:
Warning 44: this open statement shadows the value identifier sub (which is later used)
File "lib/tar_mirage.ml", line 131, characters 61-125:

This patch disables a few benign warnings in _tags