qemu_x86_64_defconfig: fix kernel header version after bump to 4.15

Closes #10831

Commit a952d24b (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>