Fix building libpng on arm CPUs.

Some arm-specific files were not part of the build.

BUG=angleproject:4026

Change-Id: Ie3534c66ebce53348a00f74b95b7f6e3859ee30f
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1866079
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Geoff Lang <geofflang@chromium.org>

third_party/libpng/BUILD.gn

@@ -24,6 +24,13 @@ static_library("libpng") {
     "src/pngwtran.c",
     "src/pngwutil.c",
   ]
+  if (target_cpu == "arm") {
+    sources += [
+      "src/arm/arm_init.c",
+      "src/arm/filter_neon.S",
+      "src/arm/filter_neon_intrinsics.c",
+    ]
+  }
   if (is_win) {
     cflags = [ "/wd4028" ]
   }