According to commit 01d6c48a ("Documentation: kselftest:
"make headers" is a prerequisite"), running the kselftests requires
to run "make headers" first.

Do that in "vmtest.sh" as well to fix the HID CI.

Link: https://lore.kernel.org/r/20230709-fix-selftests-v1-1-57d0878114cc@kernel.orgSigned-off-by: Benjamin Tissoires <bentiss@kernel.org>