Making sure HPX_WITH_ASYNC_MPI is reported properly

[hkaiser]

Fixes #5921

[codacy-production]

Coverage summary from Codacy

See diff coverage on Codacy

Coverage variation	Diff coverage
✅ -0.14%	✅ ∅

Coverage variation details

	Coverable lines	Covered lines	Coverage
Common ancestor commit (9c53b99)	206733	176197	85.23%
Head commit (941addc)	190772 (-15961)	162332 (-13865)	85.09% (-0.14%)

Coverage variation is the difference between the coverage for the head and common ancestor commits of the pull request branch: <coverage of head commit> - <coverage of common ancestor commit>

Diff coverage details

	Coverable lines	Covered lines	Diff coverage
Pull request (#6429)	0	0	∅ (not applicable)

Diff coverage is the percentage of lines that are covered by tests out of the coverable lines that the pull request added or modified: <covered lines added or modified>/<coverable lines added or modified> * 100%

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[John98Zakaria]

Thanks for fixing the issue 🥳
I tested your branch and it worked on my machine