cam: options: Make KeyValueParser::usage() private

The KeyValueParser::usage() function is meant to be called from an
OptionsParser or another KeyValueParser only. Make it private, and set
the OptionsParser class as a friend of the KeyValueParser class.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Kieran Bingham <kieran.bingham@ideasonboard.com>

src/cam/options.h

@@ -79,9 +79,11 @@ public:
 		       OptionArgument argument = ArgumentNone);
 
 	virtual Options parse(const char *arguments);
-	void usage(int indent);
 
 private:
+	friend class OptionsParser;
+	void usage(int indent);
+
 	std::map<std::string, Option> optionsMap_;
 };