Fix libpng warning.

Bug: chromium:1190892
Change-Id: I72c4c03878b3e26b7b79d4cc5975c71a21bf2d08
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3169192
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>

third_party/libpng/BUILD.gn

@@ -4,6 +4,9 @@
 
 config("libpng_config") {
   include_dirs = [ "src" ]
+  if (is_clang) {
+    cflags = [ "-Wno-tautological-constant-out-of-range-compare" ]
+  }
 }
 
 static_library("libpng") {