imx-vpu: remove kernel dependency

The imx-vpu package builds just fine without any special kernel header.

Cc: Gary Bisson <gary.bisson@boundarydevices.com>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Tested-by: Gary Bisson <gary.bisson@boundarydevices.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>