manifest: Track our own libpng

Necessary to apply patch for CVE-2019-7317

Change-Id: I8c02e55aa2b22f7996da1fc0121cf8e291eae8ae
diff --git a/default.xml b/default.xml
index ed3f94f..6382ec2 100644
--- a/default.xml
+++ b/default.xml
@@ -217,7 +217,7 @@
   <project path="external/libopus" name="platform/external/libopus" groups="pdk" remote="aosp" />
   <project path="external/libpcap" name="platform/external/libpcap" groups="pdk" remote="aosp" />
   <project path="external/libphonenumber" name="platform/external/libphonenumber" groups="pdk" remote="aosp" />
-  <project path="external/libpng" name="platform/external/libpng" groups="pdk" remote="aosp" />
+  <project path="external/libpng" name="LineageOS/android_external_libpng" groups="pdk" />
   <project path="external/libtextclassifier" name="LineageOS/android_external_libtextclassifier" groups="pdk" />
   <project path="external/libunwind" name="platform/external/libunwind" groups="pdk" remote="aosp" />
   <project path="external/libunwind_llvm" name="platform/external/libunwind_llvm" groups="pdk" remote="aosp" />