Build bc prebuilts

bc is no longer part of toybox.

Test: treehugger
Change-Id: I0820f9baa5f70452d407a378b5ca4e1c5080e160
diff --git a/build-prebuilts.sh b/build-prebuilts.sh
index cd455be..514565d 100755
--- a/build-prebuilts.sh
+++ b/build-prebuilts.sh
@@ -41,6 +41,7 @@
         ckati
         ckati_stamp_dump
         flex
+        gavinhoward-bc
         hidl-lint
         m4
         make
@@ -60,6 +61,7 @@
         acp
         aidl
         ckati
+        gavinhoward-bc
         ninja
         toybox
         zipalign