libbinder: export shared headers on Trusty

The headers under ndk/include_cpp are shared between
libbinder and libbinder_ndk, so they need to be exported
by Trusty libbinder as well.

Bug: 224644083
Test: build on Trusty
Change-Id: Ia0d314490c5f6d4fef04260d9ca920b1f6837320