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

There seems to have been merge skew between commit b2c261fa8629 ("rust:
kbuild: expand rusttest target for macros") and commit 0730422bced5
("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: 0730422bced5 ("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>

rust/Makefile

@@ -144,7 +144,7 @@ rusttestlib-kernel: private rustc_target_flags = --extern ffi \
     --extern bindings --extern uapi
 rusttestlib-kernel: $(src)/kernel/lib.rs \
     rusttestlib-bindings rusttestlib-uapi rusttestlib-build_error \
-    $(obj)/libmacros.so $(obj)/bindings.o FORCE
+    $(obj)/$(libmacros_name) $(obj)/bindings.o FORCE
 	+$(call if_changed,rustc_test_library)
 
 rusttestlib-bindings: private rustc_target_flags = --extern ffi