Fix qualcomm backend build failure

[DamonFool]

This fixes the build failure of qualcomm backend due to missing including <string>.

[  9%] Building C object backends/xnnpack/third-party/cpuinfo/CMakeFiles/cpuinfo.dir/src/x86/cache/init.c.o
In file included from /home/jiefu/executorch/backends/qualcomm/runtime/backends/QnnOpPackageManager.cpp:8:
/home/jiefu/executorch/backends/qualcomm/runtime/backends/QnnOpPackageManager.h:25:12: error: ‘std::string’ has not been declared
   25 |   bool Add(std::string qnn_op_name);
      |            ^~~
/home/jiefu/executorch/backends/qualcomm/runtime/backends/QnnOpPackageManager.h:27:12: error: ‘std::string’ has not been declared
   27 |   bool Has(std::string qnn_op_name);
      |            ^~~
/home/jiefu/executorch/backends/qualcomm/runtime/backends/QnnOpPackageManager.h:29:14: error: ‘std::string’ has not been declared
   29 |   bool Erase(std::string qnn_op_name);
      |              ^~~
/home/jiefu/executorch/backends/qualcomm/runtime/backends/QnnOpPackageManager.h:34:27: error: ‘string’ is not a member of ‘std’
   34 |   std::unordered_set<std::string> qnn_op_package_path_set_;
      |                           ^~~~~~
/home/jiefu/executorch/backends/qualcomm/runtime/backends/QnnOpPackageManager.h:11:1: note: ‘std::string’ is defined in header ‘<string>’; did you forget to ‘#include <string>’?
   10 | #include <unordered_set>
  +++ |+#include <string>
   11 | 

[pytorch-bot]

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❌ 2 New Failures, 2 Unrelated Failures

As of commit 507d908 with merge base 0b8d99f ():

NEW FAILURES - The following jobs have failed:

• pull / unittest / linux / linux-job (gh)
  RuntimeError: Command docker exec -t d472d508f8cc4dbaac2e1b86d0085b0467a8d0fd4e73f781698201dd63841b9f /exec failed with exit code 5
• pull / unittest-editable / linux / linux-job (gh)
  RuntimeError: Command docker exec -t 7f5396a3538cc7c481227b2aa207de8d1df9c3e1c76ec0df40b6d69bac8195f4 /exec failed with exit code 5

FLAKY - The following job failed but was likely due to flakiness present on trunk:

• pull / unittest-editable / macos / macos-job (gh) (similar failure)
  RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 5

BROKEN TRUNK - The following job failed but was present on the merge base:

👉 Rebase onto the `viable/strict` branch to avoid these failures

• pull / unittest / macos / macos-job (gh) (trunk failure)
  RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 5

This comment was automatically generated by Dr. CI and updates every 15 minutes.

[cccclai]

Thank you!

[DamonFool]

Thanks @cccclai .
I just pushed one more commit to fix the lintrunner check failure.
Please review again.

[DamonFool]

Thanks @Gasoonjia .