x86/cet: Fix build on newer versions of GCC

x86/cet: Fix build on newer versions of GCC

Some versions of GCC complain with:

  traps.c:405:22: error: 'get_shstk_bottom' defined but not used [-Werror=unused-function]
   static unsigned long get_shstk_bottom(unsigned long sp)
                        ^~~~~~~~~~~~~~~~
  cc1: all warnings being treated as errors

Change #ifdef to if ( IS_ENABLED(...) ) to make the sole user of
get_shstk_bottom() visible to the compiler.

Fixes: 35727551c070 ("x86/cet: Fix shskt manipulation error with BUGFRAME_{warn,run_fn}")
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Compile-tested-by: Jan Beulich <jbeulich@suse.com>
Tested-by: Marek Marczykowski-Górecki <marmarek@invisiblethingslab.com>