decimal: mark decimal_is_int() argument as const

This is preparatory commit for exposing decimals into the module API.
We'll mark the argument in the public function as const, so it would be
more accurate to have it marked as const in the internal API as well.

Part of #7228

NO_DOC=this is internal API, no user-visible changes
NO_TEST=no behavior changes
NO_CHANGELOG=no user-visible changes