configs/nezha_defconfig: use 6.1 kernel headers

The kernel used is based on 6.1-rc3, so use the recently introduced 6.1
kernel headers version to fix a build issue:

Incorrect selection of kernel headers: expected 6.0.x, got 6.1.x

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

configs/nezha_defconfig

@@ -1,5 +1,5 @@
 BR2_riscv=y
-BR2_PACKAGE_HOST_LINUX_HEADERS_CUSTOM_6_0=y
+BR2_PACKAGE_HOST_LINUX_HEADERS_CUSTOM_6_1=y
 BR2_SYSTEM_DHCP="eth0"
 BR2_ROOTFS_OVERLAY="board/nezha/overlay"
 BR2_ROOTFS_POST_IMAGE_SCRIPT="support/scripts/genimage.sh"