fixup! Rust: Tweak existing isStruct predicates

Author: paldepind

rust/ql/lib/codeql/rust/elements/internal/StructImpl.qll

@@ -48,7 +48,7 @@ module Impl {
 
     /** Holds if this struct uses struct fields. */
     pragma[nomagic]
-    predicate isStruct() { not this.isTuple() }
+    predicate isStruct() { this.getFieldList() instanceof StructFieldList }
 
     /** Holds if this struct does not have a field list. */
     predicate isUnit() { not this.hasFieldList() }