fix build against recent libosmo-abis (typo fixes)

libosmo-abis Change-Id Ifb22b5544cf06012fa529828dfdf3f0d73b07e7d
fixed spelling from existant -> existent, which breaks some of the
tests here.

Making this change catches up, but will of course fail when older
libosmo-abis versions are used.  Given the niche nature of
osmo-e1-recorder, I think it's not worth investing time into that.

Change-Id: Ib7430bf940dea33df79abe01baae670f188ff82e
1 file changed