buildroot

gst1-imx: make kernel dependency optional

The i.MX specific kernel headers are not a hard dependency. Quoting README.md:

  Without [--kernel-headers], the [ipu.h] header is not found, and elements
  using the IPU will not be built.

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