gcc-cross: Explicitly depend on linux-libc-headers

gcc-cross: Explicitly depend on linux-libc-headers

gcc-cross cannot build without linux-libc-headers but doesn't explicitly depend on
it relying on the implied dependency through libc. With cases where pieces
can be installed through sstate, we now need this explicit dependency to
ensure builds with partial sstate work.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>