configs/bananapi_m2_ultra: add kernel headers version option

configs/bananapi_m2_ultra: add kernel headers version option

The bananapi_m2_plus defconfig uses a 4.18 kernel, but doesn't specify
anything for the kernel headers version, so the Buildroot default gets
used, which currently is 4.19. Since 4.19 is newer than 4.18,
Buildroot rightfully whines:

  Incorrect selection of kernel headers: expected 4.19.x, got 4.18.x

Let's fix this by telling this defconfig to use the headers from the
kernel, and that they are of version 4.18.

Fixes:

  https://gitlab.com/buildroot.org/buildroot/-/jobs/131216938

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>