mm/ptdump: fix a -Wold-style-declaration warning

The -next commit "mm: add generic ptdump" [1] introduced a GCC
compilation warning,

mm/ptdump.c:123:1: warning: 'static' is not at beginning of declaration
[-Wold-style-declaration]
 const static struct mm_walk_ops ptdump_ops = {
 ^~~~~

[1] http://lkml.kernel.org/r/20191028135910.33253-20-steven.price@arm.com

Link: http://lkml.kernel.org/r/1572895385-29194-1-git-send-email-cai@lca.pw


Signed-off-by: Qian Cai <cai@lca.pw>
Cc: Steven Price <steven.price@arm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>