qemu_x86_64_defconfig: fix kernel header version after bump to 4.15

Closes #10831

Commit a952d24b9 (qemu_x86_64: use kernel 4.15) bumped the kernel version
but didn't update the kernel header series to match, causing build failures.

[Peter: reworded]
Signed-off-by: Ciro Santilli <ciro.santilli@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

configs/qemu_x86_64_defconfig

@@ -9,8 +9,8 @@ BR2_TARGET_GENERIC_GETTY_PORT="tty1"
 BR2_TARGET_ROOTFS_EXT2=y
 # BR2_TARGET_ROOTFS_TAR is not set
 
-# Linux headers same as kernel, a 4.11 series
-BR2_PACKAGE_HOST_LINUX_HEADERS_CUSTOM_4_11=y
+# Linux headers same as kernel, a 4.15 series
+BR2_PACKAGE_HOST_LINUX_HEADERS_CUSTOM_4_15=y
 
 # Kernel
 BR2_LINUX_KERNEL=y