Add a missing variant header in c/parallel

[caugonnet]

Description

This fixes this compilation error I have with g++15 on my local machine

      In file included from /home/caugonnet/git/caugonnet_cccl/c/parallel/src/util/runtime_policy.cpp:17:
      /home/caugonnet/git/caugonnet_cccl/c/parallel/src/util/../nvrtc/command_list.h:86:34: error: 'variant' in namespace 'std' does not name a template type
         86 | using nvrtc_linkable      = std::variant<nvrtc_ltoir, nvrtc_code>;
            |                                  ^~~~~~~
      /home/caugonnet/git/caugonnet_cccl/c/parallel/src/util/../nvrtc/command_list.h:25:1: note: 'std::variant' is defined in header '<variant>'; this is probably fixable by adding '#include <variant>'
         24 | #include <util/errors.h>
        +++ |+#include <variant>

closes

Checklist

• New or existing tests cover these changes.
• The documentation is up to date with these changes.

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[caugonnet]

/ok to test d678914

[github-actions]

🟩 CI finished in 3h 46m: Pass: 100%/26 | Total: 8h 29m | Avg: 19m 35s | Max: 3h 40m | Hits: 98%/680

• 🟩 python: Pass: 100%/22 | Total: 3h 56m | Avg: 10m 45s | Max: 20m 42s

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    🟩 Build cuda.cccl    Pass: 100%/2   | Total: 21m 51s | Avg: 10m 55s | Max: 11m 00s
    🟩 Test cuda.cccl.cooperative Pass: 100%/5   | Total:  1h 13m | Avg: 14m 37s | Max: 16m 45s
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    🟩 Test cuda.cccl.parallel Pass: 100%/5   | Total:  1h 37m | Avg: 19m 28s | Max: 20m 42s
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• 🟩 cccl_c_parallel: Pass: 100%/4 | Total: 4h 32m | Avg: 1h 08m | Max: 3h 40m | Hits: 98%/680

  🟩 cpu
    🟩 amd64              Pass: 100%/4   | Total:  4h 32m | Avg:  1h 08m | Max:  3h 40m | Hits:  98%/680   
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    🟩 12.9               Pass: 100%/4   | Total:  4h 32m | Avg:  1h 08m | Max:  3h 40m | Hits:  98%/680   
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    🟩 nvcc12.9           Pass: 100%/4   | Total:  4h 32m | Avg:  1h 08m | Max:  3h 40m | Hits:  98%/680   
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    🟩 GCC13              Pass: 100%/4   | Total:  4h 32m | Avg:  1h 08m | Max:  3h 40m | Hits:  98%/680   
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    🟩 Test               Pass: 100%/3   | Total:  4h 30m | Avg:  1h 30m | Max:  3h 40m | Hits:  98%/510   
  

👃 Inspect Changes

Modifications in project?

	Project
	CCCL Infrastructure
	CCCL Packaging
	libcu++
	CUB
	Thrust
	CUDA Experimental
	stdpar
	python
+/-	CCCL C Parallel Library
	Catch2Helper

Modifications in project or dependencies?

	Project
	CCCL Infrastructure
	CCCL Packaging
	libcu++
	CUB
	Thrust
	CUDA Experimental
	stdpar
+/-	python
+/-	CCCL C Parallel Library
	Catch2Helper

🏃‍ Runner counts (total jobs: 26)

#	Runner
17	linux-amd64-gpu-l4-latest-1
5	linux-amd64-gpu-h100-latest-1
3	linux-amd64-cpu16
1	linux-amd64-gpu-rtx2080-latest-1