Use _WIN32 instead of WIN32 to detect Windows

`WIN32` is not defined when building a 64-bit executable on Windows with Clang. `_WIN32` is the more appropriate option here.
http://web.archive.org/web/20191012035921/http://nadeausoftware.com/articles/2012/01/c_c_tip_how_use_compiler_predefined_macros_detect_operating_system contains a handy table/matrix of the options and why this is best.

rtgui/batchqueueentry.cc

@@ -50,7 +50,7 @@ BatchQueueEntry::BatchQueueEntry (rtengine::ProcessingJob* pjob, const rtengine:
 
     thumbnail = thm;
 
-#if 1 //ndef WIN32
+#if 1 //ndef _WIN32
     // The BatchQueueEntryIdleHelper tracks if an entry has been deleted while it was sitting waiting for "idle"
     bqih = new BatchQueueEntryIdleHelper;
     bqih->bqentry = this;