microkernel: events: fix undefined function error

The handler of the microkernel events are not declared properly.
This causes the public event objects to complain about
undefined functions. Fix this by declaring the function
before using it.

JIRA: ZEP-85
Change-Id: I68afe777bad66e010c92e637806a48911b2f52f4
Signed-off-by: Daniel Leung <daniel.leung@intel.com>