tracepoint.c: no empty initializer list

* ext/-test-/tracepoint/tracepoint.c (tracepoint_track_objspace_events):
  C89 disallows empty initializer lists.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@43021 b2dd03c8-39d4-4d8f-98ff-823fe69b080e