CI: Add clang-format check step

Author: felsenhower

.github/workflows/correctness_check.yaml

@@ -15,6 +15,9 @@ jobs:
       - name: Build partdiff
         run: make
 
+      - name: Check code format
+        run: clang-format --dry-run -Werror partdiff.c
+
       - name: Install Valgrind
         run: sudo apt-get update && sudo apt-get install -y valgrind