x86/sev: Drop unneeded #include

Commit:

  20f07a04 ("x86/sev: Move common memory encryption code to mem_encrypt.c")

... forgot to remove the include of virtio_config.h from mem_encrypt_amd.c
when it moved the related code to mem_encrypt.c (from where this include
subsequently got removed by a later commit).

Remove it now.

Signed-off-by: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Tom Lendacky <thomas.lendacky@amd.com>
Link: https://lore.kernel.org/r/20231010145220.3960055-3-alexander.shishkin@linux.intel.com