powerpc: Add const qual to local_read() parameter

A patch in net-next triggered a compile error on powerpc:

  include/linux/u64_stats_sync.h: In function 'u64_stats_read':
  include/asm-generic/local64.h:30:37: warning: passing argument 1 of 'local_read' discards 'const' qualifier from pointer target type

This seems reasonable to relax powerpc local_read() requirements.

Fixes: 316580b6

 ("u64_stats: provide u64_stats_t type")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: kbuild test robot <lkp@intel.com>
Acked-by: Michael Ellerman <mpe@ellerman.id.au>
Tested-by: Stephen Rothwell <sfr@canb.auug.org.au> # build only
Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>