GTP: display SAC as BASE_HEX_DEC

Bug: 10490
Change-Id: I0afe1f2ff51f2452830bfc57210dc3a298b68cbf
Reviewed-on: https://code.wireshark.org/review/4290
Reviewed-by: Pascal Quantin <pascal.quantin@gmail.com>
Reviewed-by: Anders Broman <a.broman58@gmail.com>