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/src/PSUEvent.cpp b/src/PSUEvent.cpp
index 44275a3..7813741 100644
--- a/src/PSUEvent.cpp
+++ b/src/PSUEvent.cpp
@@ -157,6 +157,8 @@
     {
         eventPollMs = static_cast<unsigned int>(pollRate * 1000);
     }
+
+    // NOLINTNEXTLINE(cppcoreguidelines-pro-type-vararg)
     fd = open(path.c_str(), O_RDONLY);
     if (fd < 0)
     {