Travis: Improve call to cppcheck

This adds:
- argument --force
- blacklists .c files that are ment to be included, only
- printing the version of cppcheck

.travis.sh

@@ -60,7 +60,19 @@ elif [[ ${MODE} = cmake-oos ]]; then
     make DESTDIR="${PWD}"/ROOT install
     find ROOT -printf "%P\n" | sort
 elif [[ ${MODE} = cppcheck ]]; then
-    cppcheck --quiet --error-exitcode=1 .
+    cppcheck --version
+    find_args=(
+        -type f \(
+            -name \*.cpp
+            -o -name \*.c
+        \)
+        -not \(  # Exclude .c files that are merely included by other files
+            -name xmltok_ns.c
+            -o -name xmltok_impl.c
+        \)
+        -exec cppcheck --quiet --error-exitcode=1 --force {} +
+    )
+    find "${find_args[@]}"
 elif [[ ${MODE} = clang-format ]]; then
     ./apply-clang-format.sh
     git diff --exit-code