rust: kbuild: use host dylib naming in rusttestlib-kernel

There seems to have been merge skew between commit b2c261fa ("rust:
kbuild: expand rusttest target for macros") and commit 0730422b
("rust: use host dylib naming convention to support macOS") ; the latter
replaced `libmacros.so` with `$(libmacros_name)` and the former added an
instance of `libmacros.so`. The former was not yet applied when the
latter was sent, resulting in a stray `libmacros.so`. Replace the stray
with `$(libmacros_name)` to allow `rusttest` to build on macOS.

Fixes: 0730422b ("rust: use host dylib naming convention to support macOS")
Signed-off-by: Tamir Duberstein <tamird@gmail.com>
Link: https://lore.kernel.org/r/20250201-fix-mac-build-again-v1-1-ca665f5d7de7@gmail.com


[ Slightly reworded title. - Miguel ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>