video_core/shader/ast: Make ExprPrinter members private

This member already has an accessor, so there's no need for it to be
public.

src/video_core/shader/ast.cpp

@@ -232,7 +232,8 @@ public:
         return inner;
     }
 
-    std::string inner{};
+private:
+    std::string inner;
 };
 
 class ASTPrinter {