Fix compilation on less recent Qt 5.x.
Fix compilation on less recent Qt 5.x.

pseudomuto/protoc-gen-doc,DengKe1994/protoc-gen-doc,mli/protoc-gen-doc,DengKe1994/protoc-gen-doc,estan/protoc-gen-doc,dr-joy/protoc-gen-doc,pseudomuto/protoc-gen-doc,DengKe1994/protoc-gen-doc,mli/protoc-gen-doc,estan/protoc-gen-doc,dr-joy/protoc-gen-doc,estan/protoc-gen-doc,DengKe1994/protoc-gen-doc,mli/protoc-gen-doc