gl_shader_decompiler: Make ExprDecompiler's GetResult() a const member function

This is only ever used to read, but not write, the resulting string, so
we can enforce this by making it a const member function.

src/video_core/renderer_opengl/gl_shader_decompiler.cpp

@@ -2335,7 +2335,7 @@ public:
         inner += expr.value ? "true" : "false";
     }
 
-    std::string& GetResult() {
+    const std::string& GetResult() const {
         return inner;
     }