Update StelFader.hpp (#4217)

Explicitly inheriting the operator = from LinearFader. Not an error but produces a lot of warnings and might potentially cause an issue in the future.

Author: pkrawczun

src/core/StelFader.hpp

@@ -139,6 +139,9 @@ class LinearFader : public StelFader
 class ParabolicFader : public LinearFader
 {
 public:
+	// Bring LinearFader::operator= into scope
+	// which is hidden due to function name hiding rules 
+	using LinearFader::operator=;
 	// Create and initialise to default
 	ParabolicFader(int _duration=1000, bool initialState=false)
 		: LinearFader(_duration, initialState)