Typo on make file

I agree that I have searched opened and closed issues to prevent duplicates.

Description

There's a / missing on https://gitlab.redox-os.org/redox-os/redox/-/blob/master/mk/prefix.mk#L22:

prefix/x86_64-unknown-redoxrust-install/x86_64-unknown-redox/include/c++

It should be:

prefix/x86_64-unknown-redox/rust-install/x86_64-unknown-redox/include/c++

make all
rm -rf "prefix/x86_64-unknown-redox/relibc-install.partial" "prefix/x86_64-unknown-redox/relibc-install"
cp -r "prefix/x86_64-unknown-redox/rust-install" "prefix/x86_64-unknown-redox/relibc-install.partial"
rm -rf "prefix/x86_64-unknown-redox/relibc-install.partial/x86_64-unknown-redox/include/"*
cp -r "prefix/x86_64-unknown-redoxrust-install/x86_64-unknown-redox/include/c++" "prefix/x86_64-unknown-redox/relibc-install.partial/x86_64-unknown-redox/include/c++"
cp: cannot stat 'prefix/x86_64-unknown-redoxrust-install/x86_64-unknown-redox/include/c++': No such file or directory
make: *** [mk/prefix.mk:22: prefix/x86_64-unknown-redox/relibc-install] Error 1

I didn't give any other information since this is probably just a typo on the make script.