For me: /usr/src/linux-headers-3.5.0-27-generic/include/linux/ipc_namespace.h
and for you possibly depends on you kernel version.
Use: apt-file search ipc_namespace.h
to locate it.
About changing it, no a simple restart will not do the trick, you will need to recompile highly possibly your kernel for the changes to happen. (And I'm not an expert in this area :( )