Fix crash in prop-types test with UnionTypeAnnotation

Author: justinanastos

lib/rules/prop-types.js

@@ -726,6 +726,10 @@ module.exports = {
           return declarePropTypesForObjectTypeAnnotation(annotation, declaredPropTypes);
         }
 
+        if (annotation.type === 'UnionTypeAnnotation') {
+          return true;
+        }
+
         const typeNode = typeScope(annotation.id.name);
 
         if (!typeNode) {