Makefile: add $(RUST_LIB) prerequisite to osxkeychain

When Rust is enabled, the git-credential-osxkeychain helper depends on Rust symbols compiled into $(RUST_LIB). While commit 522ea8ef7d ("osxkeychain: fix build with Rust") updated the linker command line to use $(LIBS), it omitted $(RUST_LIB) from the target prerequisite list. Without this prerequisite, running a parallel build ("make -j") from a clean working tree can fail because Make does not know to invoke Cargo to build libgitcore.a before linking git-credential-osxkeychain. Note that we depend explicitly on $(LIB_FILE) and $(RUST_LIB) rather than $(GITLIBS). Unlike standard Git builtins and programs like scalar (which define cmd_main() and rely on common-main.o to supply main()), git-credential-osxkeychain.c defines its own standalone int main(). If $(GITLIBS) were used, $(filter %.o,$^) in the link recipe would match both git-credential-osxkeychain.o and common-main.o, causing a duplicate symbol linking error for _main on macOS. Additionally, wrap the definitions of $(RUST_LIB) and the "rust" build target in "ifndef NO_RUST". This ensures that when NO_RUST=1 is specified, $(RUST_LIB) evaluates to empty, making the Rust dependency a clean no-op without needing intermediate variables. Signed-off-by: Shardul Natu <snatu@google.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>