Fix: compilation race issue.

Author: CorentinB

src/CMakeLists.txt

@@ -48,6 +48,9 @@ if (USE_NCNN)
   )
 endif()
 add_library(ddetect ${ddetect_SOURCES})
+if (USE_NCNN)                                                                                 
+  add_dependencies(ddetect ncnn)                                                              
+endif()
 if (USE_CAFFE)
   add_dependencies(ddetect caffe_dd)
 endif()