include: kernel: fix checkpatch line spacing warning

checkpatch misinterprets the macro that generates event data
structures as being code rather than more data.  This code has not
been changed, but rearrangement of code around it causes a false
positive when the aggregate changes are checked for style.

Add an extra line to eliminate the warning.

Signed-off-by: Peter Bigot <peter.bigot@nordicsemi.no>