Fix missing invoke header in for_each_index algorithm

[arpittkhandelwal]

This PR fixes a build regression that was recently introduced in #7215 (Implement hpx::experimental::for_each_index (P4150R0)).
Some environments were seeing compilation failures (e.g., expression cannot be used as a function inside HPX_INVOKE) because the invoke template alias wasn't fully visible to the compiler during macro expansion.
This is resolved by explicitly adding the missing #include <hpx/functional/invoke.hpp> header to libs/core/algorithms/include/hpx/parallel/algorithms/for_each_index.hpp.
Related Issues:

• Fixes build failures introduced by Implement hpx::experimental::for_each_index (P4150R0) #7215

[codacy-production]

Up to standards ✅

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Pull request overview

This PR fixes a compilation regression in hpx::experimental::for_each_index by ensuring the HPX_INVOKE machinery is fully available when for_each_index.hpp is compiled, via adding the missing hpx/functional/invoke.hpp include.

Changes:

• Add #include <hpx/functional/invoke.hpp> to for_each_index.hpp to make the invoke alias visible during HPX_INVOKE macro expansion.

[hkaiser]

@arpittkhandelwal I have pushed a possible fix to this PR. We've seen problems with HPX_INVOKE and gcc before.

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[arpittkhandelwal]

@arpittkhandelwal I have pushed a possible fix to this PR. We've seen problems with HPX_INVOKE and gcc before.

Thanks sir for the update and fixes

[hkaiser]

The remaining issues are unrelated. LGTM, thanks!