fix get prebuilts instead

Change-Id: I10730b2ec1b7bea7e7c53e205bfff8277933d006
definitions.mk: Don't fail if xmllint isn't installed

This is a workaround for xmllint currently not being installed on the
build slaves. It should be reverted when this is fixed, as it eliminates
a check for sane code.

Change-Id: I5edad10846701fde0b525237827e50d14231274d