add brew install for gtkmm4 on mac

Author: nbollom

.github/workflows/cmake_macos.yml

@@ -15,6 +15,9 @@ jobs:
         uses: jurplel/install-qt-action@v4
         with:
           version: '6.6.*'
+      - name: Install GTK
+        run: |
+          brew install gtk4mm4
       - name: Configure
         run: cmake -B macos -DCMAKE_BUILD_TYPE=Release
       - name: Build