Add clang-tools to the manifest.

cxx_extractor is needed for enabling kythe.

Bug: 201801372
Change-Id: Ib5ff5c0dd076bc3b50286eb2df3af311173b36d4

diff --git a/default.xml b/default.xml
index 5ee3912..41cfece 100644
--- a/default.xml
+++ b/default.xml

@@ -22,6 +22,7 @@
   <project path="prebuilts/gcc/linux-x86/host/x86_64-linux-glibc2.17-4.8" name="platform/prebuilts/gcc/linux-x86/host/x86_64-linux-glibc2.17-4.8" clone-depth="1" />
   <project path="prebuilts/boot-artifacts" name="platform/prebuilts/boot-artifacts" />
   <project path="prebuilts/build-tools" name="platform/prebuilts/build-tools" clone-depth="1" />
+  <project path="prebuilts/clang-tools" name="platform/prebuilts/clang-tools" clone-depth="1" />
   <project path="prebuilts/kernel-build-tools" name="kernel/prebuilts/build-tools" clone-depth="1" />
   <project path="tools/mkbootimg" name="platform/system/tools/mkbootimg" />