Fix compile on older systems with gcc 4.4.x

Signed-off-by: Waldemar Brodkorb <wbx@openadk.org>

diff -Nur gdb-7.11.1.orig//sim/common/sim-base.h gdb-7.11.1/sim/common/sim-base.h
--- gdb-7.11.1.orig//sim/common/sim-base.h	2016-02-09 21:19:39.000000000 -0600
+++ gdb-7.11.1/sim/common/sim-base.h	2016-07-08 08:02:09.000000000 -0500
@@ -72,8 +72,6 @@
 #define INVALID_INSTRUCTION_ADDRESS ((address_word)0 - 1)
 #endif
 
-/* TODO: Probably should just delete SIM_CPU.  */
-typedef struct _sim_cpu SIM_CPU;
 typedef struct _sim_cpu sim_cpu;
 
 #include "sim-module.h"