Fixed compiler warning in death: format string is not a string literal (potentially insecure) [-Wformat-security]

Building on Mac OS X 10.8.4, Apple LLVM version 4.2 (clang-425.0.28) (based on LLVM 3.2svn)

REDACTED/sp/src/game/client/death.cpp:292:8: warning: format string is not a string literal (potentially insecure) [-Wformat-security]
  Msg( killedwith+2 );
       ^~~~~~~~~~~~