inherit from QOpenGLFunctions instead of QOpenGLFunctions_2_1

The generic QOpenGLFunctions class supports OpenGL ES 2.0, the
minimum supported by Qt 5. Mixxx builds and runs fine with this;
desktop OpenGL 2.1 is not required.

src/waveform/renderers/glwaveformrenderer.h

@@ -2,9 +2,9 @@
 
 #if !defined(QT_NO_OPENGL) && !defined(QT_OPENGL_ES_2)
 
-#include <QOpenGLFunctions_2_1>
+#include <QOpenGLFunctions>
 
-class GLWaveformRenderer : protected QOpenGLFunctions_2_1 {
+class GLWaveformRenderer : protected QOpenGLFunctions {
   public:
     virtual void initializeGL() {
         initializeOpenGLFunctions();