Allow hip sources to be directly included when compiling for rocm.

[tvirolai-amd]

vLLM csrc contents are always hipified unless they are .cpp or .cc files. This causes issues when one wants to compile hip files:

If the file is named .cu then hipification either chokes on hip exclusive things or it skips it because there is nothing to hipify, meaning the .hip equivalent is not created and build fails. In addition it is confusing to call the files .cu if they are not actually valid cuda source but hip sources.

If the file is named .hip the compilation breaks due to cyclic dependency as foo.hip depends on foo.hip.

This simple change let's .hip files be excluded from hipifaction process while allowing them to automatically be recognized as hip by clang.

[github-actions]

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[mgoin]

Seems reasonable, thanks