Add a source to rule_update_info

It's possible that someone might want to override a property for a
non-syscall rule source. To assist in this, decode any source property
for rules with append/override and save it in the rule_update_info
object. For the source property only, the value for source can be
empty e.g. 'source: ' or an empty string e.g. 'source: ""'. Both of
those are considered valid but result in an empty source.

A later change will ensure that the sources match up when
appending/redefining/overriding/enabling.

Signed-off-by: Mark Stemm <mark.stemm@gmail.com>

userspace/engine/rule_loader_reader.h

@@ -66,6 +66,11 @@ public:
 	                                const char* key,
 	                                T& out,
 	                                const rule_loader::context& ctx);
+	template<typename T>
+	static void decode_optional_empty_val(const YAML::Node& item,
+	                                      const char* key,
+	                                      T& out,
+	                                      const rule_loader::context& ctx);
 
 protected:
 	virtual void read_item(rule_loader::configuration& cfg,