continue fixing nonfree elements
fixed dependency of stitching module on xfeatures2d module as optional
fixed the initModule_xfeatures2d function that was called in module stitching since it is in another namespace than the standard cv one.
Merge pull request #3163 from StevenPuttemans:fix_nonfree_items
OCL: enable clAmdFftGetVersion
Added check AmdFft version to be sure that AmdFft binaries are available
attempt to fix compilation of OpenCL cv::transpose for AMD
Merge pull request #3172 from ilya-lavrenov:tapi_transpose_amd
Merge pull request #3171 from akarsakov:amd_fft_fix