Build ziptool.

Test: treehugger
Change-Id: I9f5bf9a3428e5e3a9941f985df15c8c56b339fc0
diff --git a/build-prebuilts.sh b/build-prebuilts.sh
index 094883b..2a81e6b 100755
--- a/build-prebuilts.sh
+++ b/build-prebuilts.sh
@@ -57,6 +57,7 @@
         zip2zip
         zipalign
         ziptime
+        ziptool
     )
     SOONG_ASAN_BINARIES=(
         acp
@@ -66,6 +67,7 @@
         toybox
         zipalign
         ziptime
+        ziptool
     )
     SOONG_JAVA_LIBRARIES=(
         desugar