Add external/vixl

Change-Id: I633282d2a2ef43267edc0b5a9a250e700d713947

default.xml

@@ -240,6 +240,7 @@
   <project path="external/tremolo" name="platform/external/tremolo" groups="pdk" />
   <project path="external/v8" name="platform/external/v8" />
   <project path="external/valgrind" name="platform/external/valgrind" groups="pdk" />
+  <project path="external/vixl" name="platform/external/vixl" />
   <project path="external/webp" name="platform/external/webp" />
   <project path="external/webrtc" name="platform/external/webrtc" groups="pdk" />
   <project path="external/wpa_supplicant_8" name="platform/external/wpa_supplicant_8" groups="pdk" />