qemu_riscv64_virt_defconfig: fix linux header selection

When the kernel is built by Buildroot BR2_PACKAGE_HOST_LINUX_HEADERS_CUSTOM_X_YY
must be used and not BR2_KERNEL_HEADERS_X_YY.

Signed-off-by: Gwenhael Goavec-Merou <gwenhael.goavec-merou@trabucayre.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>