build: add write permission to sizediff GH actions job to always be able to add comments

Signed-off-by: deadprogram <ron@hybridgroup.com>

.github/workflows/sizediff.yml

@@ -10,6 +10,8 @@ concurrency:
 jobs:
   sizediff:
     runs-on: ubuntu-22.04
+    permissions:
+      pull-requests: write
     steps:
       # Prepare, install tools
       - name: Add GOBIN to $PATH