Add include for string in QnnOpPackageManager.cpp

Try to see if it resolves the build issue

Author: cccclai

backends/qualcomm/runtime/backends/QnnOpPackageManager.cpp

@@ -5,6 +5,7 @@
  * This source code is licensed under the BSD-style license found in the
  * LICENSE file in the root directory of this source tree.
  */
+#include <string>
 #include <executorch/backends/qualcomm/runtime/backends/QnnOpPackageManager.h>
 namespace executorch {
 namespace backends {