ci: add cppcheck for static analysis

.github/workflows/ci.yml

@@ -47,3 +47,13 @@ jobs:
           CC: gcc
         run: |
           make checkpatches
+  ubuntu-cppcheck:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      - name: Static analysis with cppcheck
+        run: |
+          sudo apt install cppcheck
+          cppcheck --std="c11" --error-exitcode=1 --enable=performance,portability \
+                   --force --inline-suppr --max-ctu-depth=8 -j"$(nproc)" \
+                   $(pkg-config --cflags ncurses) src/nnn.c