build: Force version.h regeneration

Force version.h generation to make sure the version number matches the
git version.  I noticed the problem after tagging v0.05-rc1 and noticing
that OSv happily reported the same version even after applying patches.

Use the special ".PHONY" target suggested by Tomek to avoid breakage
from previous attempt that is documented in commit 996fdfd ('Revert
"build: Force version.h generation"').

Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>