libipp: Convert private methods to local functions

This is a pure refactoring. No functional changes.

BUG=b:216051238,b:266139120
TEST=FEATURES=test emerge-amd64-generic libipp

Change-Id: I65c4ca1824a798455bf4eb3ec28cf5cb3b51d443
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform2/+/4270294
Reviewed-by: Benjamin Gordon <bmgordon@chromium.org>
Commit-Queue: Piotr Pawliczek <pawliczek@chromium.org>
Tested-by: Piotr Pawliczek <pawliczek@chromium.org>
NOKEYCHECK=True
GitOrigin-RevId: 7f5bdf05b84870ece0e56c3998d288710974bd26