[libc] Add AMDGPU Timing to CMake (#99603)

Summary:
`libc/benchmarks/gpu/timing/CMakeLists.txt` did not correctly build
`amdgpu` utils. This PR fixes that issue by adding `amdgpu` to the loop
that adds the correct sub directories.

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Differential Revision: https://phabricator.intern.facebook.com/D60251434

Author: jameshu15869

libc/benchmarks/gpu/timing/CMakeLists.txt

@@ -1,4 +1,4 @@
-foreach(target nvptx)
+foreach(target nvptx amdgpu)
   add_subdirectory(${target})
   list(APPEND target_gpu_timing libc.benchmarks.gpu.timing.${target}.${target}_timing)
 endforeach()