CTS: Ignore incompatibilities.txt in signatures test

The signatures verifies the API surface of libraries based on what's in prebuilts/sdk.
This is done via a glob, so with the addition of incompatibilities.txt (which is not
an API definition), it needs to be ignored explicitly.

Bug: 139128921
Test: make checkbuild
Change-Id: Id7d92d0e54bd60e3dac19d6068b64228fc840e51
Exempt-From-Owner-Approval: Approved by tobiast
2 files changed