Add unordered_map include to guard.cpp (#15077)

Fix some toolchain issue complaining `std::unordered_map` not available.

### Summary
[PLEASE REMOVE] See [CONTRIBUTING.md's Pull
Requests](https://github.com/pytorch/executorch/blob/main/CONTRIBUTING.md#pull-requests)
for ExecuTorch PR guidelines.

[PLEASE REMOVE] If this PR closes an issue, please add a `Fixes
#<issue-id>` line.

[PLEASE REMOVE] If this PR introduces a fix or feature that should be
the upcoming release notes, please add a "Release notes: <area>" label.
For a list of available release notes labels, check out
[CONTRIBUTING.md's Pull
Requests](https://github.com/pytorch/executorch/blob/main/CONTRIBUTING.md#pull-requests).

### Test plan
[PLEASE REMOVE] How did you test this PR? Please write down any manual
commands you used and note down tests that you have written if
applicable.

Author: larryliu0820

backends/cuda/runtime/guard.cpp

@@ -8,6 +8,7 @@
 
 #include <executorch/backends/cuda/runtime/guard.h>
 #include <executorch/runtime/platform/log.h>
+#include <unordered_map>
 
 namespace executorch::backends::cuda {