stackleak: Rename stackleak_track_stack to __sanitizer_cov_stack_depth

The Clang stack depth tracking implementation has a fixed name for
the stack depth tracking callback, "__sanitizer_cov_stack_depth", so
rename the GCC plugin function to match since the plugin has no external
dependencies on naming.

Link: https://lore.kernel.org/r/20250717232519.2984886-2-kees@kernel.org


Signed-off-by: Kees Cook <kees@kernel.org>