Fix two leaks reported by coverity

rtgui/previewloader.cc

@@ -168,6 +168,10 @@ PreviewLoader::PreviewLoader():
 {
 }
 
+PreviewLoader::~PreviewLoader() {
+    delete impl_;
+}
+
 PreviewLoader* PreviewLoader::getInstance()
 {
     static PreviewLoader instance_;

rtgui/previewloader.h

@@ -82,6 +82,7 @@ public:
 private:
 
     PreviewLoader();
+    ~PreviewLoader();
 
     class Impl;
     Impl* impl_;

rtgui/thumbimageupdater.cc

@@ -183,6 +183,10 @@ ThumbImageUpdater::ThumbImageUpdater():
 {
 }
 
+ThumbImageUpdater::~ThumbImageUpdater() {
+    delete impl_;
+}
+
 void ThumbImageUpdater::add(ThumbBrowserEntryBase* tbe, bool* priority, bool upgrade, ThumbImageUpdateListener* l)
 {
     // nobody listening?

rtgui/thumbimageupdater.h

@@ -89,6 +89,7 @@ public:
 private:
 
     ThumbImageUpdater();
+    ~ThumbImageUpdater();
 
     class Impl;
     Impl* impl_;