Enable cppcoreguidelines-pro-type-vararg check

We only had one usage of printf in the code that was in violation of
this rule, so replace it with iostreams, and enable the check.

Signed-off-by: Ed Tanous <edtanous@google.com>
Change-Id: Ie62165b599a996f34893aa5a3f8d1f6e6cbaf903
diff --git a/.clang-tidy b/.clang-tidy
index 9c541aa..c8fe523 100644
--- a/.clang-tidy
+++ b/.clang-tidy
@@ -187,6 +187,7 @@
 clang-analyzer-webkit.RefCntblBaseVirtualDtor,
 cppcoreguidelines-avoid-c-arrays,
 cppcoreguidelines-init-variables,
+cppcoreguidelines-pro-type-vararg,
 misc-misplaced-const,
 misc-redundant-expression,
 misc-static-assert,