kbuild: rust: clean libpin_init_internal in mrproper

When I enabled Rust compilation, I wanted to clean up its output, so I
used make mrproper. However, I was still able to find that
libpin_init_internal.so in the rust directory was not deleted, while
all other corresponding outputs were cleared.

Thus add it to the `MRPROPER_FILES` list.

Reviewed-by: Dongliang Mu <[e-mail]>
Signed-off-by: Chen Miao <[e-mail]>
Fixes: d7659acca7a3 ("rust: add pin-init crate build infrastructure")
Cc: [e-mail]
Acked-by: Nicolas Schier <[e-mail]>
Acked-by: Benno Lossin <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
[ Fixed tags and Git author as discussed. Reworded slightly. - Miguel ]
Signed-off-by: Miguel Ojeda <[e-mail]>

Makefile

···
 		  certs/x509.genkey \
 		  vmlinux-gdb.py \
 		  rpmbuild \
-		  rust/libmacros.so rust/libmacros.dylib
+		  rust/libmacros.so rust/libmacros.dylib \
+		  rust/libpin_init_internal.so rust/libpin_init_internal.dylib
 
 # clean - Delete most, but leave enough to build external modules
 #