Fix go32-nat.c build fallout from to_detach constification.

The recent constification of to_detach missed updating the forward
declaration of go32_detach, breaking the build:

 ../../src/gdb/go32-nat.c:387:1: error: conflicting types for 'go32_detach'
 ../../src/gdb/go32-nat.c:240:13: note: previous declaration of 'go32_detach' was here

go32_detach is actually defined before it's ever used, making the
forward declaration is unnecessary.  So we can just remove it instead
of updating it.  While at it, remove all others in the same situation.
Tested by building a djgpp gdb.

gdb/
2014-01-15  Pedro Alves  <palves@redhat.com>

	* go32-nat.c (go32_open, go32_close, go32_attach, go32_detach)
	(go32_resume, go32_fetch_registers, store_register)
	(go32_store_registers, go32_prepare_to_store)
	(go32_xfer_memory, go32_files_info, go32_kill_inferior)
	(go32_create_inferior, go32_can_run, go32_terminal_init)
	(go32_terminal_inferior, go32_terminal_ours): Delete forward
	declarations.