graphics/opencv: explicitely disable support for graphics/tesseract

Since the update of graphics/tesseract to 4.1.3 in 3f1cf94980fdec66bdb115c46c94635ec3b54572
the configure script of opencv now can find tesseract support,
however in a broken state.

For now, disable tesseract in graphics/opencv explicitely, so that
people building in an unclean environment with tesseract possibly
installed can still build opencv.

In the future, this will be replaced by an option toggle and fixed
properly.

PR: 259930
Reported by: rhurlin