git attribute macros not allowed in submodules

[richkadel]

This change simply moves the rust macro definition directly into the
attributes for *.rs files.

git commands that recurse from the rust toplevel tree into submodules
produce errors in clippy due to the fact that:

"Custom macro attributes can be defined only in top-level
gitattributes files"

For example, from the toplevel rust directory in a rustc development
build, try:

$ git grep "search string" --recurse-submodules

Embedded within the actual results is the error message:

[attr]rust text eol=lf whitespace=tab-in-indent,trailing-space,tabwidth=4
not allowed: src/tools/clippy/.gitattributes:1

Thank you for making Clippy better!

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changelog: none

[phansch]

@bors r+

thanks!

[bors]

📌 Commit b4736b4 has been approved by phansch

[bors]

⌛ Testing commit b4736b4 with merge d3c40a1...

[bors]

☀️ Test successful - checks-action_dev_test, checks-action_remark_test, checks-action_test
Approved by: phansch
Pushing d3c40a1 to master...