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".