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>