[Brillo] Add external/xmlrpcpp to the manifest

Adding a third-party native XML RPC library for Shill testing on Brillo.

BUG: 24335496

default.xml

@@ -307,6 +307,7 @@
   <project path="external/webp" name="platform/external/webp" groups="pdk,qcom_msm8x26" />
   <project path="external/webrtc" name="platform/external/webrtc" groups="pdk" />
   <project path="external/wpa_supplicant_8" name="platform/external/wpa_supplicant_8" groups="pdk" />
+  <project path="external/xmlrpcpp" name="platform/external/xmlrpcpp" groups="pdk" />
   <project path="external/xmlwriter" name="platform/external/xmlwriter" groups="pdk-cw-fs,pdk-fs" />
   <project path="external/xmp_toolkit" name="platform/external/xmp_toolkit" groups="pdk-cw-fs,pdk-fs" />
   <project path="external/zlib" name="platform/external/zlib" groups="pdk" />