lib: Expose rpc.pb-c.h for _CriuNotify definition

Without this, users never get a definiton for _CriuNotify and thus can't
compile anything that uses criu_notify_arg_t without invoking the protobuf
compiler themselves. The symbols are present in libcriu, so adding this as a
public include casues no issues.

Signed-off-by: Tycho Andersen <tycho.andersen@canonical.com>