Add deno-aws-sign-v4 back to database.json (denoland#954)

This was removed by denoland#837 after being added in denoland#689. It appears like the
repository owned had changed their GitHub username.

Refs:

cde8674 Removed repos which does not exists (denoland#837)
a5fd875 Adding deno-aws-sign-v4 (denoland#689)

database.json

@@ -663,6 +663,12 @@
     "repo": "deno_app_setuper",
     "desc": "A CLI to setup apps like a Create React App."
   },
+  "deno_aws_sign_v4": {
+    "type": "github",
+    "owner": "silver-xu",
+    "repo": "deno-aws-sign-v4",
+    "desc": "Generates AWS Signature V4 for AWS low-level REST APIs"
+  },
   "deno_check_thai_pid": {
     "type": "github",
     "owner": "thiti-y",