add gtkmm dependencies to GitHub Action workflow

Author: wavecommander

.github/workflows/test_linux_x64.yaml

@@ -32,7 +32,7 @@ jobs:
           channel: stable
       - run: |
           sudo apt-get update -y
-          sudo apt-get install -y ninja-build libgtk-3-dev clang cmake pkg-config build-essential
+          sudo apt-get install -y ninja-build libgtk-3-dev libgtkmm-3.0-dev libgtkmm-3.0-1t64 clang cmake pkg-config build-essential
       - run: flutter --version
       - run: lsb_release -a
       - run: clang --version