use TRACE_SET_MOD

I found this macro while refactoring and since I like consistency, I used it everywhere.

hphp/runtime/base/array/array_iterator.cpp

@@ -25,7 +25,7 @@ namespace HPHP {
 ///////////////////////////////////////////////////////////////////////////////
 // Static strings.
 
-static const Trace::Module TRACEMOD = Trace::runtime;
+TRACE_SET_MOD(runtime);
 
 static StaticString s_rewind("rewind");
 static StaticString s_valid("valid");