cgptlib: Expose GptModified as external API

SetEntry* functions are already exposed. Add GptModified function to
that list, so external users (e.g. depthcharge) can commit changes to
GPT entries and write it to the disk.

BRANCH=main
BUG=b:395630424
TEST=built FW and tested on Kano along with fastboot CL to utilize this
functionality, e.g.
  fastboot -s tcp:{dut_ip}:5554 oem set-priority:boot_a:11

Change-Id: Idb259eae80bfded59e9af177ad6751b664547cb7
Signed-off-by: Tomasz Michalec <tmichalec@google.com>
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/vboot_reference/+/6846624
Reviewed-by: Jakub "Kuba" Czapiga <czapiga@google.com>