Add const overload for tbb::combinable combine_each

[Zak-K-Abdi]

Description

Add a const overload for combine_each in combinable.h and enumerable_thread_specific.h as the ets object would also need a const overload. This is my first pull request to oneTBB and decided to work on issue #957 as it is a small change.

Fixes #957

• - git commit message contains an appropriate signed-off-by string (see CONTRIBUTING.md for details)

Type of change

Choose one or multiple, leave empty if none of the other choices apply

Add a respective label(s) to PR if you have permissions

• bug fix - change that fixes an issue
• new feature - change that adds functionality
• tests - change in tests
• infrastructure - change in infrastructure and CI
• documentation - documentation update

Tests

• added - required for new features and some bug fixes
• not needed

Documentation

• updated in # - add PR number
• needs to be updated
• not needed

Breaks backward compatibility

• Yes
• No
• Unknown

Notify the following users

List users with @ to send notifications

Other information

[pavelkumbrasev]

Hi @Zak-K-Abdi, Could you please update test for ETS and add test that won't compile without this fix?