build: Fix build with GCC 4.8.5

build: Fix build with GCC 4.8.5

GCC 4.8.5 complains:

  xtf/arch/x86/pv/traps.c: In function 'init_callbacks':
  xtf/arch/x86/pv/traps.c:126:13: error: initializer element is not constant
               .address = INIT_XEN_CALLBACK(__KERN_CS, _u(entry_EVTCHN)),
               ^
  xtf/arch/x86/pv/traps.c:126:13: error: (near initialization for 'cb[0].address')

Drop the static const.

Reported-by: Samuel Verschelde <stormi-xcp@ylix.fr>
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>