Build xz

So that we can use a prebuilt instead of a random version that may or
may not be installed on the host system.

Bug: 36130900
Test: build xz on darwin and linux
Change-Id: I541fb50f105541c46ca9070282d97747ea237921
diff --git a/build-prebuilts.sh b/build-prebuilts.sh
index 62c9905..e431959 100755
--- a/build-prebuilts.sh
+++ b/build-prebuilts.sh
@@ -48,6 +48,7 @@
         makeparallel
         ninja
         soong_zip
+        xz
         zip2zip
         zipalign
         ziptime