build/ops: make dist needs files with names > 99 characters

build/ops: make dist needs files with names > 99 characters

When running make distdir=ceph-9.0.3-1870-gfd861bb dist, a few files
have names longer than 99 characters and discarded, which then causes
the resulting tarbal to be incomplete:

tar: ceph-9.0.3-1870-gfd861bb/src/rocksdb/utilities/write_batch_with_index/write_batch_with_index_internal.cc: file name is too long (max 99); not dumped
tar: ceph-9.0.3-1870-gfd861bb/src/rocksdb/utilities/write_batch_with_index/write_batch_with_index_internal.h: file name is too long (max 99); not dumped

Use the tar-ustar format instead of the legacy v7
format (http://www.gnu.org/software/automake/manual/automake.html#Options). It
is unlikely machines with a C++11 compiler also have an antique tar
binary that would only support v7.

Signed-off-by: Loic Dachary <ldachary@redhat.com>