checkpatch: exclude forward declarations of const structs

checkpatch: exclude forward declarations of const structs

Limit checkpatch warnings for normally-const structs by excluding patterns
consistent with forward declarations.

For example, the forward declaration `struct regmap_access_table;` in a
header file currently generates a warning recommending that it is
generally declared as const; however, this would apply a useless type
qualifier in the empty declaration `const struct regmap_access_table;`,
and subsequently generate compiler warnings.

Link: https://lkml.kernel.org/r/20260331181509.1258693-1-tknelms@google.com
Signed-off-by: Taylor Nelms <tknelms@google.com>
Acked-by: Joe Perches <joe@perches.com>
Cc: Andy Whitcroft <apw@canonical.com>
Cc: Dwaipayan Ray <dwaipayanray1@gmail.com>
Cc: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>