Apply QLibrary::ExportExternalSymbolsHint only on macOS

It crashes on Debian with Qt 5.12.

Amends commit 7379a5ac52.

Change-Id: I4278833da797474eb2d0abba2185f40a709870b7
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

src/libs/extensionsystem/pluginspec.cpp

@@ -31,6 +31,7 @@
 #include "pluginmanager.h"
 
 #include <utils/algorithm.h>
+#include <utils/hostosinfo.h>
 #include <utils/qtcassert.h>
 #include <utils/stringutils.h>
 
@@ -573,6 +574,7 @@ namespace {
 PluginSpecPrivate::PluginSpecPrivate(PluginSpec *spec)
     : q(spec)
 {
+    if (Utils::HostOsInfo::isMacHost())
         loader.setLoadHints(QLibrary::ExportExternalSymbolsHint);
 }