Revert "Remove expat from the toolchain manifest."

This reverts commit 6f8740ddbc9f0f290ad437ea8f6cdc65c714609a.

Change-Id: I6f131c2dd6c90ae12956d75422547348b356d8a4
diff --git a/default.xml b/default.xml
index b0e3073..9e35ad7 100644
--- a/default.xml
+++ b/default.xml
@@ -15,6 +15,7 @@
   <project path="clang" name="toolchain/clang" />
   <project path="cloog" name="toolchain/cloog" />
   <project path="compiler-rt" name="toolchain/compiler-rt" />
+  <project path="expat" name="toolchain/expat" />
   <project path="gcc" name="toolchain/gcc" />
   <project path="gdb" name="toolchain/gdb" />
   <project path="gmp" name="toolchain/gmp" />