Improve use of &deprecated on functions.

- Don't report warnings on function definition if declaration is marked
  deprecated.
- Allow &deprecated to apply to a standalone function definition.

src/ID.h

@@ -83,6 +83,8 @@ public:
 	bool IsDeprecated() const
 		{ return FindAttr(ATTR_DEPRECATED) != 0; }
 
+	void MakeDeprecated();
+
 	void Error(const char* msg, const BroObj* o2 = 0);
 
 	void Describe(ODesc* d) const;