CI: github actions: build several popular "contrib" tools

this adds "halog", "flags" and "poll" builds. builds are done in
separate steps for better failure identification
diff --git a/.github/workflows/contrib.yml b/.github/workflows/contrib.yml
new file mode 100644
index 0000000..39556e1
--- /dev/null
+++ b/.github/workflows/contrib.yml
@@ -0,0 +1,24 @@
+name: Contrib
+
+on:
+  push:
+
+jobs:
+  build:
+
+    runs-on: ubuntu-latest
+
+    steps:
+    - uses: actions/checkout@v2
+    - name: Compile contrib/halog/halog
+      run: |
+        make contrib/halog/halog
+    - name: Compile contrib/debug/flags
+      run: |
+        make contrib/debug/flags
+    - name: Compile contrib/debug/poll
+      run: |
+        make contrib/debug/poll
+    - name: Compile contrib/hpack
+      run: |
+        make -C contrib/hpack