Update cmake.yml

.github/workflows/cmake.yml

@@ -21,6 +21,12 @@ jobs:
     steps:
     - uses: actions/checkout@v2
     
+    - name: Install system dependencies
+      if: runner.os == 'Linux'
+      run: |
+        sudo apt-get update
+        sudo apt-get -y install libgtk-3-dev
+
     - name: Configure CMake
       # Configure CMake in a 'build' subdirectory. `CMAKE_BUILD_TYPE` is only required if you are using a single-configuration generator such as make.
       # See https://cmake.org/cmake/help/latest/variable/CMAKE_BUILD_TYPE.html?highlight=cmake_build_type