list: remove redundant 'extern' for function prototypes

The 'extern' keyword is redundant for function prototypes.  list.h never
used them and new code in general is better without them.

Link: https://lkml.kernel.org/r/20250502121742.3997529-1-andriy.shevchenko@linux.intel.com


Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>