package/opencv4: BR2_PACKAGE_OPENCV4_LIB_DNN requires protobuf

Opencv4 requires protobuf library otherwise it disables DNN:
```
if(NOT HAVE_PROTOBUF)
  ocv_module_disable(opencv_dnn)
endif()
```
https://github.com/opencv/opencv/blob/4.x/modules/dnn/CMakeLists.txt#L5-L7
So let's "select BR2_PACKAGE_OPENCV4_WITH_PROTOBUF" if
BR2_PACKAGE_OPENCV4_LIB_DNN is enabled.

Signed-off-by: Giulio Benetti <giulio.benetti@benettiengineering.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

package/opencv4/Config.in

@@ -34,11 +34,14 @@ config BR2_PACKAGE_OPENCV4_LIB_DNN
 	bool "dnn"
 	# dnn needs fenv.h which is not provided by uclibc
 	depends on !BR2_TOOLCHAIN_USES_UCLIBC
+	depends on BR2_PACKAGE_PROTOBUF_ARCH_SUPPORTS
+	select BR2_PACKAGE_OPENCV4_WITH_PROTOBUF
 	help
 	  Include opencv_dnn (Deep Neural Networks) module into the
 	  OpenCV build.
 
 comment "dnn needs a glibc or musl toolchain"
+	depends on BR2_PACKAGE_PROTOBUF_ARCH_SUPPORTS
 	depends on BR2_TOOLCHAIN_USES_UCLIBC
 
 config BR2_PACKAGE_OPENCV4_LIB_FEATURES2D