libsystem_headers: export libaudio_system_headers

These headers are used in headers in libsystem_headers and must be
re-exported.

Test: modules using libsystem_headers don't have to have
libaudio_system_headers include dirs set.
Bug: 33241851

Change-Id: Ic2d842a4c97b3e3fed770249f0cc33a399f3cead