New header lib added to apex/allowed_deps.txt

libbinder_headers_platform_shared is a new header lib shared between
libbinder/libbinder_ndk. Since it is re-exported via libbinder_headers,
it should be added to allowed_deps.txt as well.

Bug: 179020493
Test: m
Test: aidl_integration_test
Change-Id: I7fe89766c8e7924edf2a3ddd69c2876611d0dd0a