Avoid casting to char* in prefetch()

Funny enough, gcc __builtin_prefetch() expects
already a void*, instead Windows's _mm_prefetch()
requires a char*.

The patch allows to remove ugly casts from caller
sites.

No functional change.

src/misc.h

@@ -28,7 +28,7 @@
 #include "types.h"
 
 const std::string engine_info(bool to_uci = false);
-void prefetch(char* addr);
+void prefetch(void* addr);
 void start_logger(bool b);
 
 void dbg_hit_on(bool b);