[coreboot] vboot/futility: Two Clang 5 warnings: address-of-packed-member and enum-conversion

[Paul Menzel]

Dear coreboot folks,

Clang 5.0 shows the warnings below. I don’t know if Clang 4.0 also
warns about these.

```
    CC            firmware/lib/vboot_api_kernel.o
firmware/lib/vboot_api_kernel.c:334:26: error: taking address of packed member 'kernel_version_tpm' of class or structure
      'VbSharedDataHeader' may result in an unaligned pointer value [-Werror,-Waddress-of-packed-member]
        if (RollbackKernelRead(&shared->kernel_version_tpm)) {
                                ^~~~~~~~~~~~~~~~~~~~~~~~~~
firmware/lib/vboot_api_kernel.c:512:16: error: implicit conversion from enumeration type 'enum vb2_nv_param' to different enumeration type
      'VbNvParam' (aka 'enum VbNvParam') [-Werror,-Wenum-conversion]
        VbNvGet(&vnc, VB2_NV_DEV_BOOT_FASTBOOT_FULL_CAP,
        ~~~~~~~       ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
2 errors generated.
```

Thanks,

Paul