OvmfPkg/EnrollDefaultKeys: suppress incorrect compiler warning

OvmfPkg/EnrollDefaultKeys: suppress incorrect compiler warning

When building OvmfPkg/EnrollDefaultKeys for IA32 with gcc-4.8 for the
DEBUG target (and possibly under other build configurations too), the
compiler incorrectly reports,

> OvmfPkg/EnrollDefaultKeys/EnrollDefaultKeys.c: In function
> 'ShellAppMain':
> OvmfPkg/EnrollDefaultKeys/EnrollDefaultKeys.c:631:10: error:
> 'SizeOfPkKek1' may be used uninitialized in this function
> [-Werror=maybe-uninitialized]
>    Status = EnrollListOfCerts (
>           ^
> OvmfPkg/EnrollDefaultKeys/EnrollDefaultKeys.c:703:12: error: 'PkKek1'
> may be used uninitialized in this function [-Werror=maybe-uninitialized]
>    FreePool (PkKek1);
>             ^
> cc1: all warnings being treated as errors

Suppress this warning, in the style suggested under
<https://bugzilla.tianocore.org/show_bug.cgi?id=607>.

Reported-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Fixes: be9470b3c91fc50436c15a76c85cdde940355b9f
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>