Always rebuild build.prop file

build.prop file was not rebuilt after changes to $(PRODUCT_PROPERTY_OVERRIDES)
Without this patch, if change I681fd92e will be merged,
the wrong date will be always in the zip file's name
(date of building build.prop and not the current date).
This patch simply forces rebuilding build.prop every time.

Change-Id: I5071072cdbb34865e32403a2e7ce0f705ff9daee