Append '\n' character to nanobench error message

This prevents the error message from inserting itself on a line before
a bench result, which can mess up parsing.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1540923003

Review URL: https://codereview.chromium.org/1540923003
diff --git a/bench/ETCBitmapBench.cpp b/bench/ETCBitmapBench.cpp
index fe6fe06..1b06112 100644
--- a/bench/ETCBitmapBench.cpp
+++ b/bench/ETCBitmapBench.cpp
@@ -81,7 +81,7 @@
 public:
     ETCBitmapBenchBase() : fPKMData(loadPKM()) {
         if (nullptr == fPKMData) {
-            SkDebugf("Could not load PKM data!");
+            SkDebugf("Could not load PKM data!\n");
         }
     }