Fix arcwaitfor__eviction tracepoint

3442c2a02d added new arc_wait_for_eviction tracepoint, which fails to
compile, when tracepoints are enabled.

The tracepoint definition begins with DEFINE_ARC_WAIT_FOR_EVICTION_EVENT
and is a multi-line definition, so this fixes the backslash
and parenthesis accordingly.

Reviewed-by: Matt Ahrens <matt@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Pavel Snajdr <snajpa@snajpa.net>
Closes #10669