intel/compiler: Use #pragma once instead of header guards
The pragma is already used elsewhere in our code so compilers building Mesa support it.
Motivation here is to reduce noise when renaming files.
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The migration is almost done, at least the rest should happen in the background. There are still a few technical difference between the old cluster and the new ones, and they are summarized in this issue. Please pay attention to the TL:DR at the end of the comment.
The pragma is already used elsewhere in our code so compilers building Mesa support it.
Motivation here is to reduce noise when renaming files.