[TASK] Make t3skin a protected extension

To ensure the backend is always in a usable and wellformed
state the t3skin extension needs to be always activated.

Releases: master
Resolves: #65528
Change-Id: I7e873a925834722158085500e2fcf2b80ef54f1e
Reviewed-on: http://review.typo3.org/37545


Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Michael Oehlhof <typo3@oehlhof.de>
Tested-by: Michael Oehlhof <typo3@oehlhof.de>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>

typo3/sysext/t3skin/composer.json

@@ -15,6 +15,7 @@
 	"extra": {
 		"typo3/cms": {
 			"Package": {
+				"protected": true,
 				"partOfFactoryDefault": true,
 				"partOfMinimalUsableSystem": true
 			}