Export libmedia headers as part of libwilhelm

sles_allinclusive.h exposes libwilhelm functionality and includes a
number of libmedia headers.  This means that to consume libwilhelm,
one needs access to libmedia's complete include path.

Bug: 27804373
Change-Id: I56a58bd96a4a8c5356e5ab8c1d458dda312f433d
Test: Brillo targets compile with this change.