imx-vpu-hantro: fix build issue

imx-vpu-hantro: fix build issue

The source code uses LINUX_VERSION_CODE at many places which constraints
the package to be built with a toolchain whose headers match the kernel.

This is a far from ideal solution, especially if one wants to use a
prebuilt toolchain (with unknown kernel header version).

So change the CFLAGS to consider the kernel build folder so that the
Linux version test actually matches the kernel built.

Signed-off-by: Gary Bisson <gary.bisson@boundarydevices.com>
Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>