logging: Document the ABI breakage in the release todo

In commit fb84f325b8e672ba7ca1a961f4ef01f4d011e33d the ABI
was broken, let's add the required entry.
diff --git a/TODO-RELEASE b/TODO-RELEASE
index 43b1e8e..72a27a4 100644
--- a/TODO-RELEASE
+++ b/TODO-RELEASE
@@ -1 +1,2 @@
 #library	what		description / commit summary line
+libosmocore	abi-change	logging: Add ability to save/print current log filters