明明已经添加了命名空间，但还是报错，百思不得其解，没办法，点进imshow函数里面，发现这个函数在highgui中声明，所以想到可能是没有将highgui的外部库添加进来，到pro里面去看，确实没有将highgui的库添加进来。所以添加以后，报错消失。
添加：
win32:
LIBS
+=
-L$$
PWD/'../../../Program
Files
(x86)/opencv/opencv-3.2.0-vc14/opencv/minGW/lib/'
-llibopencv_highgui320.dll
INCLUDEPATH
+=
$$
PWD/'../../../Program
Files
(x86)/opencv/opencv-3.2.0-vc14/opencv/minGW/
include'
DEPENDPATH
+=
$$
PWD/'../../../Program
Files
(x86)/opencv/opencv-3.2.0-vc14/opencv/minGW/
include'