Add CI for LibreSSL

When libressl-dev package is installed on Alpine Linux, LibreSSL is used
by default.

.github/workflows/libressl.yml

@@ -0,0 +1,31 @@
+name: LibreSSL
+
+on: [push, pull_request]
+
+permissions:
+  contents: read
+
+jobs:
+  libressl:
+    runs-on: ubuntu-latest
+    container:
+      image: alpine:latest
+
+    steps:
+      - name: Install dependencies
+        run: apk add binutils --no-cache build-base git readline-dev libressl-dev ncurses-dev git cmake zlib-dev libsodium-dev gnu-libiconv
+
+      - uses: actions/checkout@v6
+        with:
+          submodules: true
+
+      - name: Configure
+        run: ./configure
+
+      - name: Make
+        run: make -j $(nproc)  -C build
+
+      - name: Test
+        run: |
+          .ci/memory-leak-test.sh
+          .ci/vpntools-check.sh