opencv: Change opencv plugin to use opencv2-style includes

opencv: Change opencv plugin to use opencv2-style includes

If old opencv1-style legacy include directory is available,
this change becomes purely cosmetic (maybe will compile a bit faster).
It becomes an FTBFS fix when opencv1-style include directory is missing
(possibly because opencv package maintainer decided not to pack it).

https://bugzilla.gnome.org/show_bug.cgi?id=747705