Most certainly because the
ximgproc namespace is not included in the opencv build for ofxOpenCv?!
At least if you look into the opencv header files provided with ofxOpenCv there is no such namespace available.
Maybe @arturo can clear things up here?! As far as I remember we already had this discussion once about using opencv and cuda ... (back then the not working) solution was to use apothecary to compile opencv with the appropriate flags.
p.s.: you working in nightshift right now?
p.p.s: in case you're running on windows (ok I know you most certainly use macos) you can just ripoff the compiled opencv 3.2 library from Elliot's ofxCvMin