sim: create header namespace

The gdb/callback.h & gdb/remote-sim.h headers have nothing to do with
gdb and are really definitions for the libsim API under the sim/ tree.
While gdb uses those headers as a client, it's not specific to it.  So
create a new sim/ namespace and move the headers there.

sim/common/callback.c

@@ -35,7 +35,7 @@
 #include <time.h>
 #include <sys/types.h>
 #include <sys/stat.h>
-#include "gdb/callback.h"
+#include "sim/callback.h"
 #include "targ-vals.h"
 /* For xmalloc.  */
 #include "libiberty.h"