GHA: add a checksrc job

This job unconditionally runs checksrc on ALL .c and .h files present in
git.

checksrc.pl: fixed to look for ".checksrc" in the same directory from
where it loads the file to check so that it an be invoked like this

Closes #14625

.github/workflows/checksrc.yml

@@ -0,0 +1,29 @@
+# Copyright (C) Daniel Stenberg, <daniel@haxx.se>, et al.
+#
+# SPDX-License-Identifier: curl
+
+name: checksrc
+
+on:
+  # Trigger the workflow on push or pull requests, but only for the
+  # master branch
+  push:
+    branches:
+      - master
+      - '*/ci'
+  pull_request:
+    branches:
+      - master
+
+permissions: {}
+
+jobs:
+  all:
+
+    runs-on: ubuntu-latest
+
+    steps:
+      - uses: actions/checkout@692973e3d937129bcbf40652eb9f2f61becf3332 # v4
+
+      - name: check
+        run: git ls-files "*.[ch]" | xargs -n1 ./scripts/checksrc.pl