Make an exception for including stdarg.h in the tests

ci/no-includes.sh

@@ -1,13 +1,17 @@
 #!/usr/bin/env bash
 
 # Don't allow any system include directives in tests.
+# 
+# We make an exception for stdarg.h which is used for
+# the wrapped va_list feature. stdarg.h is available since C89
+# therefor not having this header is a sign of a bigger issue.
 
 set -eu
 cd "$(dirname "$0")/.."
 
 echo "Checking for #include directives of system headers..."
 
-grep -rn '#include\s*<.*>' bindgen-tests/tests/headers || {
+grep -rn '#include\s*<(?!stdarg).*>' bindgen-tests/tests/headers || {
     echo "Found none; OK!"
     exit 0
 }