Add m4 to the prebuilt build-tools

Bug: 117561006
Test: OUT_DIR=out DIST_DIR=dist prebuilts/build-tools/build-prebuilts.sh
Change-Id: Ic44d3640e9bbdc776a93dd2f44a767b082bc7c21
diff --git a/build-prebuilts.sh b/build-prebuilts.sh
index 1758508..e6f400f 100755
--- a/build-prebuilts.sh
+++ b/build-prebuilts.sh
@@ -46,6 +46,7 @@
         ckati
         ckati_stamp_dump
         flex
+        m4
         make
         makeparallel
         ninja