Add hidl-gen to build tools

Add hidl-gen.  It won't be used by the build, but it will be
convenient to have a prebuilt for some unbundled use cases.

Bug: 165936459
Test: OUT_DIR=out prebuilts/build-tools/build-prebuilts.sh
Change-Id: Ie6de91530238430bd0a86a12982a83769969d1d6
diff --git a/build-prebuilts.sh b/build-prebuilts.sh
index f7b2bcd..b8da7c9 100755
--- a/build-prebuilts.sh
+++ b/build-prebuilts.sh
@@ -60,6 +60,7 @@
         flex
         gavinhoward-bc
         go_extractor
+        hidl-gen
         hidl-lint
         m4
         make