header-checker: Fix opaque types comparison

This commit fixes the opaque types comparison so that opaque types with
the identical name can be treated as the identical type.

Test: ./tests/test.py MyTest.test_opaque_type_self_diff
Test: ./tests/test.py
Test: header-abi-diff does not claim that there are incompatible
changes in `liblog.so` (w/o modification).

Bug: 112404352
Change-Id: I0e96f21a65051547d70a7728fd8c03e16b543163