selftests: kvm: update .gitignore with missing file

Fixes: d5edb7f8 ("kvm: selftests: add vmx_tsc_adjust_test")
Signed-off-by: NAnders Roxell <anders.roxell@linaro.org>
Signed-off-by: NShuah Khan (Samsung OSG) <shuah@kernel.org>

tools/testing/selftests/kvm/.gitignore

 set_sregs_test
 sync_regs_test
+vmx_tsc_adjust_test