Add external/tpm2.

BUG=26191862
diff --git a/default.xml b/default.xml
index dec874b..5377db7 100644
--- a/default.xml
+++ b/default.xml
@@ -287,6 +287,7 @@
   <project path="external/tinyxml2" name="platform/external/tinyxml2" groups="pdk" />
   <project path="external/tlsdate" name="platform/external/tlsdate" />
   <project path="external/toybox" name="platform/external/toybox" groups="pdk" />
+  <project path="external/tpm2" name="platform/external/tpm2" />
   <project path="external/tremolo" name="platform/external/tremolo" groups="pdk" />
   <project path="external/v8" name="platform/external/v8" groups="pdk" />
   <project path="external/valgrind" name="platform/external/valgrind" groups="pdk" />