Bug 1818652: Unguard an include in nsTraceRefcnt.cpp, to address a...

Bug 1818652: Unguard an include in nsTraceRefcnt.cpp, to address a non-unified-build compile error. r=mccr8

Also move the include to the top of the list, to its proper sorted-include-list
location, and insert a blank line after the .cpp file's own .h file, to match
common convention in mozilla code.

Differential Revision: https://phabricator.services.mozilla.com/D170837