Ignore private symbols from ABI check

I didn't have any issue about this, it's just something that came out when discussing the ABI checker with Dodji, the libabigail author.

One thing to note is that with this change, we're effectively checking the ABI of the Sdk, not of the Platform. It should be fine though, because as far as I know the Sdk has more stuff than the Platform, but what they both have is identical in both.

Report is a bit leaner though (no more "ignored function changes for libgtk3.so"), and each check should consume a bit less memory. (probably still not enough for LLVM?)