Makefile: Add the dependency for BUILT_OTATOOLS_PACKAGE.

$ lunch aosp_marlin-userdebug
$ m otatools-package
$ echo "TEST" >> device/google/marlin/sepolicy/certs/app.x509.pem
$ m otatools-package

out/target/product/marlin/otatools.zip now contains an updated
app.x509.pem file.

Test: see above
Change-Id: I78a165e12a07c1b86a671a7ee1863a97fdea18c5