Fixes compilation issue of class event_impl being not defined when AF_DISABLE_CPU_ASYNC macro is defined.

Description

• Is this a new feature or a bug fix?: Bug Fix
• Why these changes are necessary: Fixes compilation issue for a possible option in compilation
• Potential impact on specific hardware, software or backends: Same impact on all platforms on CPU backend
• New functions and their functionality: None
• Can this PR be backported to older versions?: Yes
• Future changes not implemented in this PR.: None

Fixes: #3551

Changes to Users

• No build options added
• No action required by the user

Checklist

• Rebased on latest master
• Code compiles
• Tests pass
• Functions added to unified API
• Functions documented