osxkeychain: fix build with Rust

osxkeychain: fix build with Rust

Without NO_RUST defined, the varint encoder/decoder lives in the
RUST_LIB, which needs to be linked. Symptom:

cc [... -o contrib/credential/osxkeychain/git-credential-osxkeychain [...]
Undefined symbols for architecture x86_64:
  "_decode_varint", referenced from:
      _read_untracked_extension in libgit.a[x86_64][63](dir.o)
      _read_untracked_extension in libgit.a[x86_64][63](dir.o)
      _read_one_dir in libgit.a[x86_64][63](dir.o)
      _read_one_dir in libgit.a[x86_64][63](dir.o)
      _load_cache_entry_block in libgit.a[x86_64][174](read-cache.o)
  "_encode_varint", referenced from:
      _write_untracked_extension in libgit.a[x86_64][63](dir.o)
      _write_untracked_extension in libgit.a[x86_64][63](dir.o)
      _write_untracked_extension in libgit.a[x86_64][63](dir.o)
      _write_one_dir in libgit.a[x86_64][63](dir.o)
      _write_one_dir in libgit.a[x86_64][63](dir.o)
      _do_write_index in libgit.a[x86_64][174](read-cache.o)
ld: symbol(s) not found for architecture x86_64

While it is curious why these functions are needed at all (osxkeychain
does not read or write the index), the compile error is a real problem.

Instead of trying to play games to add `GITLIBS` while filtering out
`common-main.o`, replace the `$(LIB_FILE) $(EXTLIBS)` construct with the
much shorter `$(LIBS)` construct that _already_ filters out
`common-main.o` and adds the Rust library when needed.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>