More angle brackets for system includes for third_party deps

"In general anything in third_party should probably use angle brackets"

A follow-on from https://skia-review.googlesource.com/c/skia/+/474756

Change-Id: I4a2b07e1625952637c090a82971ab2ab426db034
Bug: skia:12541
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/475098
Reviewed-by: Brian Salomon <bsalomon@google.com>
diff --git a/tests/EncodeTest.cpp b/tests/EncodeTest.cpp
index 0d5ea8a..c5f7937 100644
--- a/tests/EncodeTest.cpp
+++ b/tests/EncodeTest.cpp
@@ -20,7 +20,7 @@
 #include "include/encode/SkWebpEncoder.h"
 #include "include/private/SkImageInfoPriv.h"
 
-#include "png.h"
+#include <png.h>
 
 #include <algorithm>
 #include <string>