build: harden GitHub workflow permissions

Grant pull-requests write permission to the labeler workflow and
read-only to everything else.

Signed-off-by: Alex Low <aleksandrosansan@gmail.com>
[ wrap to 80 columns and fix wrong author as requested by author itself ]
Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>

.github/workflows/kernel.yml

@@ -7,6 +7,10 @@ on:
       - 'include/kernel-*'
       - 'package/kernel/**'
       - 'target/linux/generic/**'
+
+permissions:
+  contents: read
+
 jobs:
   determine_targets:
     name: Set targets