DO NOT MERGE: Revert "remove external/libnl-headers from the manifest"

This reverts commit fd1e90b3b1.
libnl_2 needs to stay in AOSP for now for compatibility with
GPL test builds.
Change-Id: I85365fb4b8931a998e564d30800a06343a47fdd8

default.xml

@@ -154,6 +154,7 @@
   <project path="external/libnfc-nci" name="platform/external/libnfc-nci" groups="pdk" />
   <project path="external/libnfc-nxp" name="platform/external/libnfc-nxp" groups="pdk" />
   <project path="external/libnl" name="platform/external/libnl" />
+  <project path="external/libnl-headers" name="platform/external/libnl-headers" groups="pdk" />
   <project path="external/libogg" name="platform/external/libogg" />
   <project path="external/libpcap" name="platform/external/libpcap" />
   <project path="external/libphonenumber" name="platform/external/libphonenumber" />