Build cxx_extractor

(cxx_extractor generates .kzip files used to cross-reference source
code).

Bug: 121267023
Test: run prebuilts/clang-tools/build-prebuilts.sh
Change-Id: Iaf9561ca05622a3134e0ea2bf96bdedd27df8409
diff --git a/build-prebuilts.sh b/build-prebuilts.sh
index 653c833..7791408 100755
--- a/build-prebuilts.sh
+++ b/build-prebuilts.sh
@@ -56,6 +56,7 @@
 
 # Targets to be built
 SOONG_BINARIES=(
+    "cxx_extractor"
     "header-abi-linker"
     "header-abi-dumper"
     "header-abi-diff"