Fix fallout from gdb/20413's fix (x32: linux_ptrace_test_ret_to_nx: Cannot PTRACE_PEEKUSER)

Fixes, on NIOS GNU/Linux:

  In file included from
  /scratch/mbilal/nois-lite/src/gdb-trunk/gdb/gdbserver/../nat/linux-ptrace.c:26:0:
  /scratch/mbilal/nois-lite/src/gdb-trunk/gdb/gdbserver/../gregset.h:27:23:
  error: unknown type name 'gregset_t'
   #define GDB_GREGSET_T gregset_t
			 ^

Fix this by including sys/procfs.h directly.  We shouldn't really be
including a gdb-only header in a gdb/nat/ file, anyway.  Whoops.

gdb/ChangeLog:
2016-08-11  Pedro Alves  <palves@redhat.com>

	PR gdb/20413
	* nat/linux-ptrace.c: Include <sys/procfs.h> instead of
	"gregset.h".