configs/qemu_nios2: add custom kernel headers version

When this qemu nios2 defconfig has been added, the insternal toolchain
used for testing was build with Linux kernel headers 4.9, so
BR2_PACKAGE_HOST_LINUX_HEADERS_CUSTOM_4_9 was missing in the defconfig.

Signed-off-by: Romain Naour <romain.naour@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

configs/qemu_nios2_10m50_defconfig

@@ -5,3 +5,6 @@ BR2_LINUX_KERNEL_CUSTOM_VERSION_VALUE="4.9.6"
 BR2_LINUX_KERNEL_DEFCONFIG="10m50"
 BR2_LINUX_KERNEL_CONFIG_FRAGMENT_FILES="board/qemu/nios2-10m50/linux-4.9.fragment"
 BR2_TARGET_ROOTFS_INITRAMFS=y
+
+# Linux headers same as kernel, a 4.9 series
+BR2_PACKAGE_HOST_LINUX_HEADERS_CUSTOM_4_9=y