lockdep: Mark chain_hlock_class_idx() with __maybe_unused

lockdep: Mark chain_hlock_class_idx() with __maybe_unused

When chain_hlock_class_idx() is unused, it prevents kernel builds with
clang, `make W=1` and CONFIG_WERROR=y, CONFIG_LOCKDEP=y and
CONFIG_PROVE_LOCKING=n:

kernel/locking/lockdep.c:435:28: error: unused function 'chain_hlock_class_idx' [-Werror,-Wunused-function]

Fix this by marking it with __maybe_unused.

See also commit 6863f5643dd7 ("kbuild: allow Clang to find unused static
inline functions for W=1 build").

[Boqun: add more config information of the error]

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
Link: https://lore.kernel.org/r/20241209170810.1485183-1-andriy.shevchenko@linux.intel.com