manual-kernel: Add bc to buildNativeInputs.

As timeconst.pl was replaced by a bc script in the merge window for kernel 3.9,
we also need bc during build time, for more information about the reasons,
please have a look at the following commit:

torvalds/linux@70730bca1331fc50c3caacaea00439de1325bd6e

Signed-off-by: aszlig <aszlig@redmoonstudios.org>