hurd: add missing include

Fixes

../../gdb/gnu-nat.c:2522:14: error: ‘target_gdbarch’ was not declared in this scope; did you mean ‘target_detach’?
 2522 |    paddress (target_gdbarch (), memaddr), pulongest (len),

gdb/Changelog:

	* gnu-nat.c: Include "gdbarch.h".

gdb/gnu-nat.c

@@ -64,6 +64,7 @@ extern "C"
 #include "language.h"
 #include "target.h"
 #include "gdbsupport/gdb_wait.h"
+#include "gdbarch.h"
 #include "gdbcmd.h"
 #include "gdbcore.h"
 #include "gdbthread.h"