Fix compilation for x86_64-linux target using libc with -dFPC_USE_LIBC option

packages/rtl-extra/src/unix/ipc.pp

@@ -18,7 +18,7 @@ unit ipc;
 interface
 
 Uses
-{$ifdef FPC_USE_LIBCX}
+{$ifdef FPC_USE_LIBC}
   initc,
 {$endif}
   BaseUnix,UnixType;