ci: pack-ify our custom CodeQL queries and enable them in Actions

Unlike LGTM, the CodeQL Action requires the custom queries to have their
own qlpack.yml file, so let's provide one.

.github/codeql-config.yml

@@ -6,4 +6,7 @@ name: "CodeQL config"
 disable-default-queries: false
 
 queries:
-  - uses: ./.github/codeql-custom.qls
+  - name: Enable possibly useful queries which are disabled by default
+    uses: ./.github/codeql-custom.qls
+  - name: systemd-specific CodeQL queries
+    uses: ./.lgtm/cpp-queries/

.lgtm/cpp-queries/qlpack.yml

@@ -0,0 +1,11 @@
+---
+# vi: ts=2 sw=2 et syntax=yaml:
+# SPDX-License-Identifier: LGPL-2.1-or-later
+
+library: false
+name: systemd/cpp-queries
+version: 0.0.1
+dependencies:
+  codeql/cpp-all: "*"
+  codeql/suite-helpers: "*"
+extractor: cpp