wiggle: fix lifetime in as_str_ptr

crates/wiggle/src/lib.rs

@@ -536,7 +536,7 @@ impl<'a> GuestPtr<'a, str> {
 impl<'a> GuestPtr<'a, [u8]> {
     /// Returns a raw pointer to the string represented by a `[u8]` without
     /// validating whether each u8 is a utf-8 codepoint.
-    pub fn as_str_ptr(&self) -> GuestPtr<str> {
+    pub fn as_str_ptr(&self) -> GuestPtr<'a, str> {
         GuestPtr::new(self.mem, self.pointer)
     }
 }