test_resize function has some bug when use opencv lib

[tuuzhang]

at first,thank you for the project.when I try to use the opencv lib,there are some small bug finded:
./src/image.c: In function ‘test_resize’:
./src/image.c:1464:9: error: ‘k’ undeclared (first use in this function)

[ArtyZe]

@tuuzhang ， Hi， actually the project does not support opencv so far, but it's enough for you to use without opencv, are you really need opencv to do some special work in the project?