Fix gdbserver SPU build

Fix gdbserver SPU build

Ref: https://sourceware.org/ml/gdb-patches/2015-08/msg00675.html

 gdbserver/spu-low.c: In function 'spu_request_interrupt':
 gdbserver/spu-low.c:639: error: incompatible type for argument 1 of 'ptid_get_lwp'

gdb/gdbserver/ChangeLog:
2015-08-24  Pedro Alves  <palves@redhat.com>

	* spu-low.c (spu_request_interrupt): Use lwpid_of instead of
	ptid_get_lwp.