Make pbtombstone a host tool.

This is preparation for the next patch, which adds host-side
symbolization capabilities to pbtombstone.

Bug: 328531087
Change-Id: Id5813ae6b121af784643b1ed76084e49fdca118b
diff --git a/target/product/base_system.mk b/target/product/base_system.mk
index 74ed82d..d4a49ab 100644
--- a/target/product/base_system.mk
+++ b/target/product/base_system.mk
@@ -424,6 +424,7 @@
     lpdump \
     mke2fs \
     mkfs.erofs \
+    pbtombstone \
     resize2fs \
     sgdisk \
     sqlite3 \