package/opencv3: use external protobuf instead of bundled one

Since version 3.3.0 and its commit
https://github.com/opencv/opencv/commit/1c4c58d1b5b7430463281e8c8072e04245f561b0

,
opencv3 bundles its own copy of protobuf. Instead of using the bundled
protobuf library, this commit disables building protobuf
(BUILD_PROTOBUF=OFF) and instead uses the protobuf Buildroot package
(using the WITH_PROTOBUF=ON/OFF option).

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>