libenca: Move build host tool to PRE_BUILD_HOOKS

The make_hash host tool, should be built during a PRE_BUILD_HOOKS and
not a POST_CONFIGURE_HOOKS.

Signed-off-by: Maxime Hadjinlian <maxime.hadjinlian@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>