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>

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