C++: Make some methods static to match the C API

Context::package_version() and Context::lib_version() do not access
context state and should be static. However, leave the logging
related methods alone for now, as making them static would entail
making the callback data a global static, since the C API lacks
destroy notification callbacks.

bindings/cxx/include/libsigrokcxx/libsigrokcxx.hpp

@@ -261,9 +261,9 @@ public:
 	/** Create new context */
 	static shared_ptr<Context> create();
 	/** libsigrok package version. */
-	string package_version();
+	static string package_version();
 	/** libsigrok library version. */
-	string lib_version();
+	static string lib_version();
 	/** Available hardware drivers, indexed by name. */
 	map<string, shared_ptr<Driver> > drivers();
 	/** Available input formats, indexed by name. */