Fix clang 12 build by explicitly installing libunwind-12 (#46)

Author: andreasbuhr

.github/workflows/cmake.yml

@@ -142,7 +142,7 @@ jobs:
         wget https://apt.llvm.org/llvm.sh
         chmod +x llvm.sh
         sudo ./llvm.sh 12
-        sudo apt-get install libc++-12-dev libc++abi-12-dev
+        sudo apt-get install libc++-12-dev libc++abi-12-dev libunwind-12-dev
 
     - name: Install g++ 10
       id: install_gcc_10