gst1-plugins-bad: add missing depends on C++ for the OpenCV plugin

OpenCV requires C++ support, so we shouldn't select it without
depending on C++ support.

Fixes:
http://autobuild.buildroot.org/results/71f/71f2d8a267a0d159521407a9a2330e68d756c4b3/build-end.log.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>

package/multimedia/gst1-plugins-bad/Config.in

@@ -400,10 +400,14 @@ config BR2_PACKAGE_GST1_PLUGINS_BAD_PLUGIN_NEON
 
 config BR2_PACKAGE_GST1_PLUGINS_BAD_PLUGIN_OPENCV
 	bool "opencv"
+	depends on BR2_INSTALL_LIBSTDCPP
 	select BR2_PACKAGE_OPENCV
 	help
 	  GStreamer OpenCV Plugins
 
+comment "opencv plugin requires a toolchain with C++ support"
+	depends on !BR2_INSTALL_LIBSTDCPP
+
 config BR2_PACKAGE_GST1_PLUGINS_BAD_PLUGIN_OPUS
 	bool "opus"
 	select BR2_PACKAGE_OPUS