configs/qemu_ppc_virtex_ml507: kernel build needs mkimage

Following ffbe46a5 ("linux: simplify
LINUX_BUILD_CMDS"), the Linux kernel build for
qemu_ppc_virtex_ml507_defconfig builds an image format that needs
mkimage.

Fixes:

  https://gitlab.com/buildroot.org/buildroot/-/jobs/199339544



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