SIFT has different namespace in Linux and Apple

Linux: cv::xfeatures2d::SIFT

Apple: cv::SIFT

Closes #6 (closed)

src/descriptors/sift/descriptor_sift.h

@@ -48,7 +48,7 @@ inline void DescriptorSIFT::defineDescriptor(const ParamsBasePtr _params)
 {
     DescriptorParamsSIFTPtr params_ptr = std::static_pointer_cast<DescriptorParamsSIFT>(_params);
 
-    descriptor_ = cv::xfeatures2d::SIFT::create(params_ptr->nfeatures,
+    descriptor_ = cv::SIFT::create(params_ptr->nfeatures,
     		params_ptr->nOctaveLayers,
 			params_ptr->contrastThreshold,
 			params_ptr->edgeThreshold,