Include gdb_proc_service.h in x86-linux-nat.h

Include gdb_proc_service.h in x86-linux-nat.h

$ make check-headers CHECK_HEADERS="x86-linux-nat.h"
...
../../binutils-gdb/gdb/x86-linux-nat.h:29:8: error: 'ps_err_e' does not name a type
 extern ps_err_e x86_linux_get_thread_area (pid_t pid, void *addr,
        ^

gdb:

2017-01-06  Yao Qi  <yao.qi@linaro.org>

	* x86-linux-nat.h: Include gdb_proc_service.h.