glsl: Address rest of feedback

src/shader_recompiler/backend/glsl/emit_context.h

@@ -136,6 +136,8 @@ public:
         code += '\n';
     }
 
+    [[nodiscard]] bool StageInitializesVaryings() const noexcept;
+
     std::string header;
     std::string code;
     VarAlloc var_alloc;