Dummy declarations for commonly used classes in DSP code

Signed-off-by: falkTX <falktx@falktx.com>

libs/juce-plugin/JucePluginMain.cpp

@@ -40,3 +40,10 @@
 #else
  #error Invalid configuration
 #endif
+
+#if JUCE_AUDIOPROCESSOR_NO_GUI
+// commonly used classes in DSP code
+namespace juce {
+Colour::Colour(juce::uint32) noexcept {}
+}
+#endif