ProcessStub: run setupUnixInferior() only on Unices

setupUnixInferior() is meant to run only Unix systems, in the same way
setupWindowsInferior() is meant only for Windows. Hence, guard its
whole body within Q_OS_UNIX.

Followup of commit 075876e8b2.

Change-Id: Ia3482c18e30d1044cc022c24413bcac7663e3829
Reviewed-by: Marcus Tillmanns <marcus.tillmanns@qt.io>

src/tools/process_stub/main.cpp

@@ -277,6 +277,7 @@ void onInferiorStarted()
 
 void setupUnixInferior()
 {
+#ifdef Q_OS_UNIX
     if (debugMode) {
         qCInfo(log) << "Debug mode enabled";
 #ifdef Q_OS_DARWIN
@@ -296,6 +297,7 @@ void setupUnixInferior()
         });
 #endif
     }
+#endif
 }
 
 void setupWindowsInferior()