编辑.pro文件时，似乎没有重新生成Makefile。
运行qmake，然后重试。
您还可以检查deck.cpp是否已编译；在构建目录中有一个deck.o吗？

It looks like the Makefile was not regenerated when you edited the .pro file. Run qmake and try again. You could also check if the deck.cpp is compiled or not; is there a deck.o in the build directory ?