Restore libxml2 to Yosemite

Ok, so here is the deal. I was working on two projects at the same time and I mistakenly:

rm -rf /usr/lib/libxml2.2.dylib

I have another copy of libxml2 thankfully (the other project, homebrew install libxml2). That copy is in /usr/local/opt/libxml/lib. For the life of me I can not get cp /usr/local/opt/libxml2/lib/libxml2.2.dylib /usr/lib to work. It always says:

/usr/lib/libxml2.2.dylib Read-only Filesystem

Please can anyone help? I have tried cp -S, -R, -F and I can't get it to cp or mv. I just need to write that lib back and I should be good to go. I don't know what I was thinking when I did that. Running Yosemite 10.10.2, Late-2011 mbp. Libxml2 from homebrew is 2.9.2 I think if it makes a difference.