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>