meson: drop separate version library

meson: drop separate version library

When building `libgit.a` we link it against a `libgit_version.a` library
that contains the version information that we inject at build time. The
intent of this is to avoid rebuilding all of `libgit.a` whenever the
version changes. But that wouldn't happen in the first place, as we know
to just rebuild the files that depend on the generated "version-def.h"
file.

This is an artifact of an earlier version of the Meson build infra that
didn't ultimately land. We didn't yet have "version-def.h", and instead
injected the version via preprocessor directives. And here we would have
rebuilt all of `libgit.a` indeed in case the version changes, because
the preprocessor directive applied to all files.

Stop building the separate library and instead add "version-def.h" to
the list of source files directly.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>