cpu: Guard cpu_{save,load}() definitions

A few targets already managed to implement cpu_save() and cpu_load()
without defining CPU_SAVE_VERSION that causes them to be registered.

Guard the prototypes with CPU_SAVE_VERSION to avoid this happening again
until all targets are converted to VMState or QIDL.

Signed-off-by: Andreas Färber <afaerber@suse.de>

include/qemu-common.h

@@ -280,8 +280,10 @@ bool tcg_enabled(void);
 void cpu_exec_init_all(void);
 
 /* CPU save/load.  */
+#ifdef CPU_SAVE_VERSION
 void cpu_save(QEMUFile *f, void *opaque);
 int cpu_load(QEMUFile *f, void *opaque, int version_id);
+#endif
 
 /* Unblock cpu */
 void qemu_cpu_kick_self(void);