simpleperf: don't expose libunwindstack in OfflineUnwinder.h.

It is to build etm decode library happen to include
OfflineUnwinder.h, but don't link with libunwindstack.

Bug: 135204414
Test: run simpleperf_unit_test.
Change-Id: I924498e7315b50ff44f85445f23c4be342bd39bd
6 files changed