Add xsl files to .editorconfig

Change-Id: Ic5e44527eb821b42b607cc805d6f2e6883a74755
Reviewed-on: https://gerrit.libreoffice.org/40982Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Samuel Mehrbrodt <Samuel.Mehrbrodt@cib.de>

.editorconfig

@@ -23,3 +23,7 @@ trim_trailing_whitespace = false
 # Patch files
 [*.patch.*]
 trim_trailing_whitespace = false
+
+# XSL files
+[*.xsl]
+indent_style = tab