Mark log2fapprox as const.

sdrbase/dsp/spectrumvis.cpp

@@ -1098,7 +1098,7 @@ void SpectrumVis::webapiUpdateSpectrumSettings(
 // so we can use the following approximation to get a good speed-up for both compilers:
 // https://www.vplesko.com/posts/replacing_log2f.html
 // https://www.vplesko.com/assets/replacing_log2f/main.c.txt
-float SpectrumVis::log2fapprox(float x)
+float SpectrumVis::log2fapprox(float x) const
 {
     // IEEE 754 representation constants.
     const int32_t mantissaLen = 23;

sdrbase/dsp/spectrumvis.h

@@ -259,7 +259,7 @@ private:
     void handleScalef(Real scalef);
     void handleWSOpenClose(bool openClose);
     void handleConfigureWSSpectrum(const QString& address, uint16_t port);
-    float log2fapprox(float x);
+    float log2fapprox(float x) const;
 
     static void webapiFormatSpectrumSettings(SWGSDRangel::SWGGLSpectrum& response, const SpectrumSettings& settings);
     static void webapiUpdateSpectrumSettings(