travis/gitlab/azure: Run cppcheck in parallel

This takes ages to run single-threaded. Adjust it to use all available
processors.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/.gitlab-ci.yml b/.gitlab-ci.yml
index a4f8a71..08bdf81 100644
--- a/.gitlab-ci.yml
+++ b/.gitlab-ci.yml
@@ -103,7 +103,7 @@
   tags: [ 'all' ]
   stage: testsuites
   script:
-    - cppcheck --force --quiet --inline-suppr .
+    - cppcheck -j$(nproc) --force --quiet --inline-suppr .
 
 # search for TODO within source tree
 grep TODO/FIXME/HACK: