efi_selftest: efi_selftest_snp: Fix warning when building with clang

efi_selftest: efi_selftest_snp: Fix warning when building with clang

When building with clang, we see a warning:
lib/efi_selftest/efi_selftest_snp.c:63:18: error: field dhcp_hdr within
'struct dhcp' is less aligned than 'struct dhcp_hdr' and is usually due
to 'struct dhcp' being packed, which can lead to unaligned accesses
[-Werror,-Wunaligned-access]
when building lib/efi_selftest/efi_selftest_snp.c. Resolve this error by
packing struct dhcp_hdr as well, as the only place it is used also is
packed.

Signed-off-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>