Fix "beneath" conversion on AIX

Fix "beneath" conversion on AIX

GDB build on AIX is broken according to BuildBot:

  ../../binutils-gdb/gdb/aix-thread.c: In member function 'virtual void aix_thread_target::mourn_inferior()':
  ../../binutils-gdb/gdb/aix-thread.c:1735:34: error: 'beneath' cannot be used as a function
     target_ops *beneath = beneath ();
				    ^
This obvious commit fixes it.  There's apparently another issue
breaking the build there, but that's unrelated.

gdb/ChangeLog:
2018-06-14  Sergio Durigan Junior  <sergiodj@redhat.com>

	* aix-thread.c (aix_thread_target::xfer_partial): Use
	"beneath" as a method.