ci: fix Graphviz detection in cmake

.gitlab-ci.yml

@@ -52,7 +52,9 @@ build:windows:
   script:
     - mkdir build
     - cd build
-    - cmake ..
+    # 64 Bit Graphviz Builds for Windows:
+    # https://ci.appveyor.com/project/ellson/graphviz-pl238/
+    - cmake .. -DGraphviz_ROOT="C:/Program Files/Graphviz 2.41.20190716.1839"
     - cmake --build .
   tags:
     - win10