luci: add macOS filename to .gitignore

Using GitHub Desktop on macOS.
When viewing my cloned files in
Finder, it creates a macOS file that
GitHub wants to commit unless I add
it to the .gitignore for the repository.
Would be beneficial to have ready
in file for future situations.

Signed-off-by: Joshua Criss <owlsy@outlook.com.au>

.gitignore

@@ -18,3 +18,4 @@ modules/luci-compat/src/plural_formula.h
 docs/**/*.*
 **eslint.config**
 !eslint.config.mjs
+.DS_Store