ArmVirtPkg: Add FileExplorerLib.inf to the QEMU dsc files

Because SecureBootConfigDxe use FileExplorerLib now, but
FileExplorerLib is not in the dsc files of the package
that use SecureBootConfigDxe. Now add it to pass build.

Cc: Laszlo Ersek <lersek@redhat.com>
Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Eric Dong <eric.dong@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Dandan Bi <dandan.bi@intel.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Eric Dong <eric.dong@intel.com>