Fix for gdb.base/stap-probe.c

On PowePC, the test fails on a compile error:

  /../binutils-gdb-current/gdb/testsuite/gdb.base/stap-probe.c:107:1: error: expected '=', ',', ';', 'asm' or 'attribute' before 'use_xmm_reg'
  107 | use_xmm_reg (int val)
  | ^~~~~~~~~~~

Where the source code for stap-probe.c is:

  static const char * __attribute__((noinline)) ATTRIBUTE_NOCLONE
  use_xmm_reg (int val)                         <-- line 107
  {
  ...

The issue is the ATTRIBUTE_NOCLONE is not defined as an attribute as
expected.  The #define for ATTRIBUTE_NOCLONE can be found in
../lib/attributes.h.

This patch adds the missing include statement for the definition of
ATTRIBUTE_NOCLONE.

The patch has been tested and verified on a Power10 system.