Add external/libunwind to manifest.

default.xml

@@ -136,6 +136,7 @@
   <project path="external/libppp" name="platform/external/libppp" />
   <project path="external/libselinux" name="platform/external/libselinux" groups="pdk" />
   <project path="external/libsepol" name="platform/external/libsepol" groups="pdk" />
+  <project path="external/libunwind" name="platform/external/libunwind" groups="pdk" />
   <project path="external/libusb" name="platform/external/libusb" />
   <project path="external/libusb_aah" name="platform/external/libusb_aah" groups="libusb_aah" />
   <project path="external/libusb-compat" name="platform/external/libusb-compat" />