sim: dv-sockser: push module init prototype down

Pull out the duplicated dv_sockser_install prototype from the tconfig.in
files and put it in the one place it gets used -- sim-module.c.  This is
still arguably incorrect, but it's better than the status quo where the
tconfig.in has to include header files and duplicate the dv-sockser func.
The tconfig header is meant to be simple and contain a target defines.

include/gdb/remote-sim.h

@@ -60,9 +60,10 @@ typedef enum {
 } SIM_RC;
 
 
-/* The bfd struct, as an opaque type.  */
+/* Some structs, as opaque types.  */
 
 struct bfd;
+struct host_callback_struct;
 
 
 /* Main simulator entry points.  */