GN: Change libpng to be a group rather than a static_library, since it has no sources.

BUG=416829
R=brettw@chromium.org

Review URL: https://codereview.chromium.org/598233002

Cr-Commit-Position: refs/heads/master@{#296566}

third_party/libpng/BUILD.gn

@@ -74,7 +74,7 @@ if (is_win) {
     public_deps = [ ":libpng_sources" ]
   }
 } else {
-  static_library("libpng") {
+  group("libpng") {
     public_deps = [ ":libpng_sources" ]
   }
 }