package/opencv4: BR2_PACKAGE_OPENCV4_LIB_OBJDETECT requires BR2_PACKAGE_OPENCV4_LIB_DNN

Opencv4 library objdetect needs library dnn, otherwise it throws:
```
-- Module opencv_objdetect disabled because opencv_dnn dependency can't be resolved!
```
So let's "select BR2_PACKAGE_OPENCV4_LIB_DNN" if
BR2_PACKAGE_OPENCV4_LIB_OBJDETECT is enabled.

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

package/opencv4/Config.in

@@ -153,14 +153,21 @@ config BR2_PACKAGE_OPENCV4_LIB_ML
 
 config BR2_PACKAGE_OPENCV4_LIB_OBJDETECT
 	bool "objdetect"
+	depends on !BR2_TOOLCHAIN_USES_UCLIBC # dnn support
+	depends on BR2_PACKAGE_PROTOBUF_ARCH_SUPPORTS # dnn support
 	# opencv_core dependency is already enabled
 	select BR2_PACKAGE_OPENCV4_LIB_CALIB3D
+	select BR2_PACKAGE_OPENCV4_LIB_DNN
 	select BR2_PACKAGE_OPENCV4_LIB_IMGPROC
 	select BR2_PACKAGE_OPENCV4_LIB_ML
 	help
 	  Include opencv_objdetect (object detection) module into the
 	  OpenCV build.
 
+comment "objdetect needs a glibc or musl toolchain"
+	depends on BR2_PACKAGE_PROTOBUF_ARCH_SUPPORTS
+	depends on BR2_TOOLCHAIN_USES_UCLIBC
+
 config BR2_PACKAGE_OPENCV4_LIB_PHOTO
 	bool "photo"
 	select BR2_PACKAGE_OPENCV4_LIB_IMGPROC