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>

configs/qemu_riscv64_virt_defconfig

@@ -11,7 +11,7 @@ BR2_TARGET_GENERIC_GETTY=y
 BR2_TARGET_GENERIC_GETTY_PORT="ttyS0"
 
 # Linux headers same as kernel, a 4.20 series
-BR2_KERNEL_HEADERS_4_20=y
+BR2_PACKAGE_HOST_LINUX_HEADERS_CUSTOM_4_20=y
 
 # Kernel
 BR2_LINUX_KERNEL=y