Compile test_ffmpeg_video_reader failed.

[longyiluo]

After building decord, run "make test_ffmpeg_video_reader" and got the following errors.
libdecord.so is already build successfully in the build folder.

root@ee8adfd337e2:/home/longyiluo/decord/build# make test_ffmpeg_video_reader
[ 93%] Built target decord
[ 96%] Linking CXX executable test_ffmpeg_video_reader
/usr/bin/ld: CMakeFiles/test_ffmpeg_video_reader.dir/tests/cpp/video/test_ffmpeg_video_reader.cc.o: in function main': /home/longyiluo/decord/tests/cpp/video/test_ffmpeg_video_reader.cc:19: undefined reference to decord::GetVideoReader(std::__cxx11::basic_string<char, std::char_traits, std::allocator >, DLContext, int, int, int, int)'
collect2: error: ld returned 1 exit status
make[3]: *** [CMakeFiles/test_ffmpeg_video_reader.dir/build.make:91: test_ffmpeg_video_reader] Error 1
make[2]: *** [CMakeFiles/Makefile2:107: CMakeFiles/test_ffmpeg_video_reader.dir/all] Error 2
make[1]: *** [CMakeFiles/Makefile2:114: CMakeFiles/test_ffmpeg_video_reader.dir/rule] Error 2
make: *** [Makefile:177: test_ffmpeg_video_reader] Error 2