libdw: Make sure id_path can contain max number of build id bytes.

libdw: Make sure id_path can contain max number of build id bytes.

The MAX_BUILD_ID_BYTES is fairly large (64), while normally build-ids
are only 20 bytes long. But if we would encounter a jumbo build-id we
should have enough room to construct the full build-id path.

We used to substract 2 bytes from the max, because 2 chars are used
as subdir. But that should be 1 (2 hex chars is just one 8 bit byte).

Signed-off-by: Mark Wielaard <mark@klomp.org>