WFM modulator: Windows build: fixed compile error with MinGW

plugins/channeltx/modwfm/wfmmod.cpp

@@ -215,7 +215,7 @@ void WFMMod::pullAF(Complex& sample)
     }
 }
 
-void WFMMod::calculateLevel(Real& sample)
+void WFMMod::calculateLevel(const Real& sample)
 {
     if (m_levelCalcCount < m_levelNbSamples)
     {

plugins/channeltx/modwfm/wfmmod.h

@@ -326,7 +326,7 @@ private:
 
     void apply();
     void pullAF(Complex& sample);
-    void calculateLevel(Real& sample);
+    void calculateLevel(const Real& sample);
     void openFileStream();
     void seekFileStream(int seekPercentage);
 };