Enable PSA so that this example can run

This example requires PSA to be enabled in order for the PSA APIs to be
available for use. This example uses PSA Crypto APIs and thus requires
the target to have the PSA label in order to make those PSA APIs
available.

Author: Patater

mbed_app.json

@@ -1,7 +1,8 @@
 {
     "target_overrides": {
         "*": {
-             "platform.stdio-convert-newlines": true
+             "platform.stdio-convert-newlines": true,
+             "target.extra_labels_add": ["PSA"]
         }
     }
 }