BaseTools/VolInfo: Correct buffer for GenCrc32 tool

BaseTools/VolInfo: Correct buffer for GenCrc32 tool

If the guided section was encoded with GenCrc32 tool the resulting
'EFI_GUID_DEFINED_SECTION.DataOffset' field points to the start of
the meaningfull data that follows the CRC32 value.
But if we want to decode the section with GenCrc32 tool we need to
provide a buffer that includes the CRC32 value itself.

Signed-off-by: Konstantin Aladyshev <aladyshev22@gmail.com>
Reviewed-by: Liming Gao <gaoliming@byosoft.com.cn>
Reviewed-by: Bob Feng <bob.c.feng@intel.com>