Fix gcc 4.5 compile error/warning in HrPwPlot.cpp

A couple of nits in the code were picked up by
gcc-4.5; const in a function returning non-const
and an unneccessary initialisation of a QList calling
the constructor directly.

Fixes #236.

src/HrPwPlot.cpp

@@ -35,7 +35,7 @@
 #include <qwt_data.h>
 
 
-static const inline double
+static inline double
 max(double a, double b) { if (a > b) return a; else return b; }
 
 HrPwPlot::HrPwPlot(MainWindow *mainWindow, HrPwWindow *hrPwWindow) :
@@ -100,7 +100,6 @@ HrPwPlot::HrPwPlot(MainWindow *mainWindow, HrPwWindow *hrPwWindow) :
     r_mrk1->attach(this);
     r_mrk2->attach(this);
 
-    zoneLabels = QList <HrPwPlotZoneLabel *>::QList();
     shade_zones = true;
 }