effects/showfps: Add destructor

The GLTexture class is forward declared and the ShowFpsEffect class has
a QScopedPointer<GLTexture> field. We either need to include the
kwinglutils.h header file or add a destructor that does nothing and
define it in the cpp file, where kwinglutils.h is included.

src/effects/showfps/showfps.cpp

@@ -51,6 +51,10 @@ ShowFpsEffect::ShowFpsEffect()
     reconfigure(ReconfigureAll);
 }
 
+ShowFpsEffect::~ShowFpsEffect()
+{
+}
+
 void ShowFpsEffect::reconfigure(ReconfigureFlags)
 {
     ShowFpsConfig::self()->read();

src/effects/showfps/showfps.h

@@ -33,6 +33,8 @@ class ShowFpsEffect
     Q_PROPERTY(QColor textColor READ configuredTextColor)
 public:
     ShowFpsEffect();
+    ~ShowFpsEffect() override;
+
     void reconfigure(ReconfigureFlags) override;
     void prePaintScreen(ScreenPrePaintData& data, std::chrono::milliseconds presentTime) override;
     void paintScreen(int mask, const QRegion &region, ScreenPaintData& data) override;