tools: add coverage to ignored files

This adds the coverage directory to the .gitignore file.

PR-URL: #28626
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Masashi Hirano <shisama07@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Сковорода Никита Андреевич <chalkerx@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>

.gitignore

@@ -105,6 +105,7 @@ _UpgradeReport_Files/
 # coverage related
 /gcovr
 /build
+/coverage
 
 # === Rules for XCode artifacts ===
 *.xcodeproj