Add external/libpng

This exercises the need for additional flags for external modules.

Test: build/bazel/ci/bp2build.sh
Change-Id: I0110640a0639577e7f40f53676d94fb7973c3233
diff --git a/android/bazel.go b/android/bazel.go
index 8c63204..26b4abc 100644
--- a/android/bazel.go
+++ b/android/bazel.go
@@ -296,6 +296,7 @@
 		"external/libcxx":                                    Bp2BuildDefaultTrueRecursively,
 		"external/libcxxabi":                                 Bp2BuildDefaultTrueRecursively,
 		"external/libevent":                                  Bp2BuildDefaultTrueRecursively,
+		"external/libpng":                                    Bp2BuildDefaultTrueRecursively,
 		"external/lz4/lib":                                   Bp2BuildDefaultTrue,
 		"external/lzma/C":                                    Bp2BuildDefaultTrueRecursively,
 		"external/mdnsresponder":                             Bp2BuildDefaultTrueRecursively,
@@ -303,9 +304,9 @@
 		"external/pcre":                                      Bp2BuildDefaultTrueRecursively,
 		"external/protobuf":                                  Bp2BuildDefaultTrueRecursively,
 		"external/python/six":                                Bp2BuildDefaultTrueRecursively,
-		"external/selinux/libsepol":                          Bp2BuildDefaultTrueRecursively,
 		"external/scudo":                                     Bp2BuildDefaultTrueRecursively,
 		"external/selinux/libselinux":                        Bp2BuildDefaultTrueRecursively,
+		"external/selinux/libsepol":                          Bp2BuildDefaultTrueRecursively,
 		"external/zlib":                                      Bp2BuildDefaultTrueRecursively,
 		"external/zstd":                                      Bp2BuildDefaultTrueRecursively,
 		"frameworks/base/media/tests/MediaDump":              Bp2BuildDefaultTrue,