gitlint: Add an exception for GDB backtraces

This way the lint won't complain about oversized lines inside GDB backtraces.
It will patch the initial " #<number>" pattern along with the terminating
":<line>".

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/5049>

.gitlint

@@ -14,3 +14,7 @@ min-length=10
 # Ensure every title starts with a prefix
 [title-match-regex]
 regex=^[\w]+[\w, -\\/]*[\w]+: .*
+
+# Ignore GDB backtraces
+[ignore-body-lines]
+regex=^ \#\d(.*):\d+$