Adding another missing docs dependency

Author: emprice

.github/workflows/docs.yml

@@ -19,7 +19,7 @@ jobs:
 
       - name: Install documentation dependencies
         run: |
-          sudo apt install opencl-c-headers ocl-icd-opencl-dev
+          sudo apt install opencl-c-headers ocl-icd-opencl-dev doxygen
           pip3 install -U -r $GITHUB_WORKSPACE/doc/requirements.txt
           pip3 install -U --ignore-installed $GITHUB_WORKSPACE