Bug 1423798 - Remove headers included for backwards compat in nsString.h. r=njn on a CLOSED TREE

Remove the headers included for "backwards compatibility" and just include them
where required.

xpcom/base/LogModulePrefWatcher.cpp

@@ -8,6 +8,7 @@
 
 #include "mozilla/Logging.h"
 #include "mozilla/Preferences.h"
+#include "nsMemory.h"
 #include "nsString.h"
 #include "nsXULAppAPI.h"
 #include "base/process_util.h"