selftests: hid: fix vmtests.sh not running make headers

According to commit 01d6c48a828b ("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/[e-mail]
Signed-off-by: Benjamin Tissoires <[e-mail]>

tools/testing/selftests/hid/vmtest.sh

···
 	cd "${kernel_checkout}"
 
 	${make_command} olddefconfig
+	${make_command} headers
 	${make_command}
 }