fix: build kernel before compiling packages in build-package workflow

The kernel was never compiled, causing luci-app-tailscale to fail with
missing modules.builtin and .config files.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.github/workflows/build-package.yml

@@ -61,6 +61,7 @@ jobs:
         cd openwrt
         make tools/install -j$(nproc)
         make toolchain/install -j$(nproc)
+        make target/linux/compile -j$(nproc) V=s
 
     - name: Build APK packages
       run: |