OvmfPkg/PlatformPei: sync AmdSevInitialize() definition with declaration

OvmfPkg/PlatformPei: sync AmdSevInitialize() definition with declaration

"Platform.h" declares the AmdSevInitialize() function without EFIAPI, but
the definition in "AmdSev.c" includes EFIAPI.

GCC toolchains without LTO do not catch this error because "AmdSev.c" does
not include "Platform.h"; i.e. the declaration used by callers such as
"Platform.c" is not actually matched against the function definition at
build time.

With LTO enabled, the mismatch is found -- however, as a warning only, due
to commit f8d0b9662993 ("BaseTools GCC5: disable warnings-as-errors for
now", 2016-08-03).

Include the header in the C file (which turns the issue into a hard build
error on all GCC toolchains), plus sync the declaration from the header
file to the C file.

There's been no functional breakage because AmdSevInitialize() takes no
parameters.

Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Brijesh Singh <brijesh.singh@amd.com>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Fixes: 13b5d743c87a22dfcd94e8475d943dee5712b62d
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Brijesh Singh <brijesh.singh@amd.com>
Reviewed-by: Jaben Carsey <jaben.carsey@intel.com>
Acked-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>