Add Ubuntu to GitHub Actions build matrix

Install required X11 development packages (libx11-dev, libxi-dev) for
Linux builds alongside existing GTK3 dependencies.

Author: lijy91

.github/workflows/build.yml

@@ -12,9 +12,12 @@ jobs:
       matrix:
         os:
           - macos-latest
+          - ubuntu-latest
         include:
           - os: macos-latest
             platform: macos
+          - os: ubuntu-latest
+            platform: linux
 
     runs-on: ${{ matrix.os }}
 
@@ -27,7 +30,7 @@ jobs:
           cmake --version
           if [ "${{ matrix.platform }}" = "linux" ]; then
             sudo apt-get update
-            sudo apt-get install -y ninja-build libgtk-3-dev
+            sudo apt-get install -y ninja-build libgtk-3-dev libx11-dev libxi-dev
           fi
 
       - name: Configure CMake