hdrgen: remove trailing whitespace on attribute declarations

[ljmf00]

Fixes Issue 22353.

Signed-off-by: Luís Ferreira contact@lsferreira.net

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	22353	enhancement	Header generation is producing trailing whitespace on attribute declarations

Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub run digger -- build "master + dmd#13121"

[thewilsonator]

please rebase to restart CI