gccrs: Add getter for non const lifetime object

gcc/rust/ChangeLog:

	* hir/tree/rust-hir-item.h (SelfParam::get_lifetime): Add getter
	for non const lifetime object

Signed-off-by: Ryutaro Okada <[email protected]>

Author: sakupan102

gcc/rust/hir/tree/rust-hir-item.h

@@ -402,6 +402,8 @@ struct SelfParam
 
   const Lifetime &get_lifetime () const { return lifetime.value (); }
 
+  Lifetime &get_lifetime () { return lifetime.value (); }
+
   std::string as_string () const;
 
   location_t get_locus () const { return locus; }